Interpret -1 to select.poll.poll as blocking, especially under libuv

Why are we bothering to run cares on PyPy? It has several known-bad tests, and it sometimes crashes the interpreter. We don't recommend or support it now that we have dnspython.

Fix #1127 by interpreting -1 as None under libuv for select.poll.poll; should probably start having Timeout do that itself.

Cherry pick 1ef16f38491000f4fdafdc01dc809d30fc785867

Add a GreenletTree for more organized, clearer output of greenlets

It organizes things by the spawning greenlet, if possible.

Example output::

```
<greenlet.greenlet object at 0x10753a9b0>
 :    Running:
 :      File "/tmp/t.py", line 38, in <module>
 :        print("\n".join(format_run_info()))
 :      File "//src/gevent/util.py", line 99, in format_run_info
 :        _format_greenlet_info(lines)
 :      File "//src/gevent/util.py", line 132, in _format_greenlet_info
 :        lines.extend(tree.format_lines(details=True))
 :      File "//src/gevent/util.py", line 265, in format_lines
 :        for l in self._render(tree)]
 :      File "/-main/src/gevent/util.py", line 310, in _render
 :        self.__render_tb(tree, 'Running:', self.greenlet.gr_frame)
 :      File "//src/gevent/util.py", line 281, in __render_tb
 :        tb = ''.join(traceback.format_stack(frame))
 :    Greenlet Locals:
 :      Local <type 'gevent._local.local'> at 0x10759cec0
 :        {'foo': 42}
 +--- <Greenlet "Greenlet-0" at 0x107405cb0: _run>; finished with value <Greenlet "Greenlet-4" at 0x10
 :          Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Spawned at:
 :            File "/tmp/t.py", line 1, in <module>
 :              from gevent.util import GreenletTree
 |    +--- <Greenlet "Greenlet-4" at 0x10780b260: _run>; finished
 :                Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                Spawned at:
 :                  File "/tmp/t.py", line 1, in <module>
 :                    from gevent.util import GreenletTree
 :                  File "/tmp/t.py", line 12, in t2
 :                    def t2():
 +--- <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Parent: <greenlet.greenlet object at 0x10753a9b0>
 :          Running:
 :            File "/Users/jmadden/Projects/GithubSources/gevent-main/src/gevent/hub.py", line 673, in run
 :              loop.run()
 +--- <Greenlet "Greenlet-1" at 0x10780b368: _run>; finished with value <Greenlet "Greenlet-5" at 0x10
 :          Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Spawned at:
 :            File "/tmp/t.py", line 1, in <module>
 :              from gevent.util import GreenletTree
 |    +--- <Greenlet "Greenlet-5" at 0x10780b578: _run>; finished
 :                Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                Spawned at:
 :                  File "/tmp/t.py", line 1, in <module>
 :                    from gevent.util import GreenletTree
 :                  File "/tmp/t.py", line 12, in t2
 :                    def t2():
 +--- <Greenlet "Greenlet-2" at 0x10780b470: _run>; finished with value <Greenlet "Greenlet-6" at 0x10
 :          Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :          Spawned at:
 :            File "/tmp/t.py", line 1, in <module>
 :              from gevent.util import GreenletTree
 :          Spawn Tree Locals
 :          {'stl': 'STL'}
 |    +--- <Greenlet "Greenlet-6" at 0x10780b680: _run>; finished with value <Greenlet "Greenlet-7" at 0x10
 :                Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                Spawned at:
 :                  File "/tmp/t.py", line 1, in <module>
 :                    from gevent.util import GreenletTree
 :                  File "/tmp/t.py", line 21, in t3
 :                    def t3():
 |         +--- <Greenlet "Greenlet-7" at 0x10780b788: _run>; finished
 :                      Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
 :                      Spawned at:
 :                        File "/tmp/t.py", line 1, in <module>
 :                          from gevent.util import GreenletTree
 :                        File "/tmp/t.py", line 21, in t3
 :                          def t3():
 :                        File "/tmp/t.py", line 12, in t2
 :                          def t2():
 +--- <Greenlet "Greenlet-3" at 0x10780b890: _run>; finished with value [<gevent.util.GreenletTree obj
            Parent: <Hub at 0x10753a550 select default pending=0 ref=0>
            Spawned at:
              File "/tmp/t.py", line 1, in <module>
                from gevent.util import GreenletTree
```

Include locals in the debugging output.

Also let greenlets have a name.

Some tweaks to the local implementation to speed it up by 3-4%.

Speed up Greenlet creation on CPython

Two ways: store tuples instead of _frame objects and use direct access
to two of the attributes of the CPython frame objects.

Benchmarks:

+------------------------+-----------------+------------------------------+
| Benchmark              | spawn_27_master | spawn_27_tuple2              |
+========================+=================+==============================+
| eventlet sleep         | 9.12 us         | 8.77 us: 1.04x faster (-4%)  |
+------------------------+-----------------+------------------------------+
| gevent spawn           | 14.5 us         | 13.2 us: 1.10x faster (-9%)  |
+------------------------+-----------------+------------------------------+
| gevent sleep           | 1.63 us         | 1.86 us: 1.14x slower (+14%) |
+------------------------+-----------------+------------------------------+
| geventpool spawn       | 30.4 us         | 23.6 us: 1.29x faster (-22%) |
+------------------------+-----------------+------------------------------+
| geventpool sleep       | 4.30 us         | 4.55 us: 1.06x slower (+6%)  |
+------------------------+-----------------+------------------------------+
| geventpool join        | 1.70 us         | 1.83 us: 1.08x slower (+8%)  |
+------------------------+-----------------+------------------------------+
| gevent spawn kwarg     | 16.5 us         | 13.5 us: 1.22x faster (-18%) |
+------------------------+-----------------+------------------------------+
| geventpool spawn kwarg | 30.5 us         | 23.9 us: 1.27x faster (-22%) |
+------------------------+-----------------+------------------------------+

Not significant (7): eventlet spawn; geventraw spawn; geventraw sleep;
none spawn; eventlet spawn kwarg; geventraw spawn kwarg; none spawn
kwarg

+------------------------+-----------------+------------------------------+
| Benchmark              | spawn_36_master | spawn_36_tuple2              |
+========================+=================+==============================+
| gevent spawn           | 13.2 us         | 11.9 us: 1.12x faster (-10%) |
+------------------------+-----------------+------------------------------+
| gevent sleep           | 1.71 us         | 1.90 us: 1.11x slower (+11%) |
+------------------------+-----------------+------------------------------+
| geventpool spawn       | 19.9 us         | 15.9 us: 1.25x faster (-20%) |
+------------------------+-----------------+------------------------------+
| geventpool sleep       | 3.54 us         | 3.75 us: 1.06x slower (+6%)  |
+------------------------+-----------------+------------------------------+
| geventpool spawn kwarg | 20.3 us         | 15.9 us: 1.27x faster (-22%) |
+------------------------+-----------------+------------------------------+
| geventraw spawn kwarg  | 5.80 us         | 6.10 us: 1.05x slower (+5%)  |
+------------------------+-----------------+------------------------------+

Not significant (9): eventlet spawn; eventlet sleep; geventraw spawn;
geventraw sleep; none spawn; geventpool join; eventlet spawn kwarg;
gevent spawn kwarg; none spawn kwarg

+------------------+-------------------+------------------------------+
| Benchmark        | spawn_pypy_master | spawn_pypy_tuple2            |
+==================+===================+==============================+
| eventlet spawn   | 30.5 us           | 28.9 us: 1.05x faster (-5%)  |
+------------------+-------------------+------------------------------+
| eventlet sleep   | 3.39 us           | 3.19 us: 1.06x faster (-6%)  |
+------------------+-------------------+------------------------------+
| gevent spawn     | 9.89 us           | 17.2 us: 1.73x slower (+73%) |
+------------------+-------------------+------------------------------+
| gevent sleep     | 3.14 us           | 3.99 us: 1.27x slower (+27%) |
+------------------+-------------------+------------------------------+
| geventpool spawn | 12.3 us           | 20.1 us: 1.63x slower (+63%) |
+------------------+-------------------+------------------------------+

Not significant (1): geventpool sleep

+------------------------+---------------+-------------------------------+
| Benchmark              | spawn_36_13a1 | spawn_36_tuple2               |
+========================+===============+===============================+
| eventlet spawn         | 14.0 us       | 13.2 us: 1.06x faster (-6%)   |
+------------------------+---------------+-------------------------------+
| gevent spawn           | 4.25 us       | 11.9 us: 2.79x slower (+179%) |
+------------------------+---------------+-------------------------------+
| gevent sleep           | 2.78 us       | 1.90 us: 1.46x faster (-32%)  |
+------------------------+---------------+-------------------------------+
| geventpool spawn       | 10.4 us       | 15.9 us: 1.52x slower (+52%)  |
+------------------------+---------------+-------------------------------+
| geventpool sleep       | 5.52 us       | 3.75 us: 1.47x faster (-32%)  |
+------------------------+---------------+-------------------------------+
| geventraw spawn        | 2.56 us       | 5.09 us: 1.99x slower (+99%)  |
+------------------------+---------------+-------------------------------+
| geventraw sleep        | 738 ns        | 838 ns: 1.14x slower (+14%)   |
+------------------------+---------------+-------------------------------+
| geventpool join        | 3.94 us       | 1.75 us: 2.25x faster (-56%)  |
+------------------------+---------------+-------------------------------+
| gevent spawn kwarg     | 5.50 us       | 12.1 us: 2.19x slower (+119%) |
+------------------------+---------------+-------------------------------+
| geventpool spawn kwarg | 11.3 us       | 15.9 us: 1.41x slower (+41%)  |
+------------------------+---------------+-------------------------------+
| geventraw spawn kwarg  | 3.90 us       | 6.10 us: 1.56x slower (+56%)  |
+------------------------+---------------+-------------------------------+

Not significant (4): eventlet sleep; none spawn; eventlet spawn kwarg; none spawn kwarg

The eventlet, sleep, join and raw tests serve as controls, so we can see
that there's up to ~10% variance between most runs anyway.

CPython 3.6 shows the least variance so those 10-20% improvement
numbers are probably fairly close.

PyPy sadly gets *slower* with this change for reasons that are utterly
unclear.

Compared to 1.3a1 (last benchmark) we're still up to 2-3x slower.

Creation of a raw greenlet shows 2.66us on CPython 3.6.4 vs the 3.65us
I reported in #755.

I get this on 3.6.4:

.....................
eventlet spawn: Mean +- std dev: 12.2 us +- 1.2 us
.....................
eventlet sleep: Mean +- std dev: 16.3 us +- 0.8 us
.....................
gevent spawn: Mean +- std dev: 13.1 us +- 1.1 us
.....................
gevent sleep: Mean +- std dev: 10.4 us +- 0.8 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (2.73 us) is 17% of the mean (16.1 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn: Mean +- std dev: 16.1 us +- 2.7 us
.....................
geventpool sleep: Mean +- std dev: 11.2 us +- 0.5 us
.....................
geventraw spawn: Mean +- std dev: 4.95 us +- 0.42 us
.....................
geventraw sleep: Mean +- std dev: 7.34 us +- 0.28 us
.....................
none spawn: Mean +- std dev: 1.98 us +- 0.05 us
.....................
geventpool join: Mean +- std dev: 6.59 us +- 0.25 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (1.28 us) is 10% of the mean (12.7 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

eventlet spawn kwarg: Mean +- std dev: 12.7 us +- 1.3 us
.....................
gevent spawn kwarg: Mean +- std dev: 14.6 us +- 1.2 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (2.81 us) is 17% of the mean (17.0 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn kwarg: Mean +- std dev: 17.0 us +- 2.8 us
.....................
geventraw spawn kwarg: Mean +- std dev: 6.11 us +- 0.45 us
.....................
none spawn kwarg: Mean +- std dev: 2.22 us +- 0.07 us

And this on 2.7.14:

.....................
WARNING: the benchmark result may be unstable
* the standard deviation (2.10 us) is 11% of the mean (18.4 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

eventlet spawn: Mean +- std dev: 18.4 us +- 2.1 us
.....................
eventlet sleep: Mean +- std dev: 23.1 us +- 0.8 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (4.39 us) is 25% of the mean (17.3 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

gevent spawn: Mean +- std dev: 17.3 us +- 4.4 us
.....................
gevent sleep: Mean +- std dev: 10.3 us +- 0.5 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (3.92 us) is 16% of the mean (24.7 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn: Mean +- std dev: 24.7 us +- 3.9 us
.....................
geventpool sleep: Mean +- std dev: 13.5 us +- 0.9 us
.....................
geventraw spawn: Mean +- std dev: 6.91 us +- 0.49 us
.....................
geventraw sleep: Mean +- std dev: 8.95 us +- 0.30 us
.....................
none spawn: Mean +- std dev: 2.21 us +- 0.04 us
.....................
geventpool join: Mean +- std dev: 7.93 us +- 0.28 us
.....................
eventlet spawn kwarg: Mean +- std dev: 17.4 us +- 1.3 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (4.11 us) is 27% of the mean (15.1 us)
* the maximum (24.2 us) is 60% greater than the mean (15.1 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

gevent spawn kwarg: Mean +- std dev: 15.1 us +- 4.1 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (4.74 us) is 18% of the mean (26.8 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn kwarg: Mean +- std dev: 26.8 us +- 4.7 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (959 ns) is 12% of the mean (8.00 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventraw spawn kwarg: Mean +- std dev: 8.00 us +- 0.96 us
.....................
none spawn kwarg: Mean +- std dev: 2.48 us +- 0.06 us

Partial PyPy results:

.........
WARNING: the benchmark result may be unstable
* the standard deviation (5.77 us) is 18% of the mean (32.5 us)
* the maximum (52.2 us) is 61% greater than the mean (32.5 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

eventlet spawn: Mean +- std dev: 32.5 us +- 5.8 us
.........
eventlet sleep: Mean +- std dev: 39.9 us +- 2.4 us
.........
WARNING: the benchmark result may be unstable
* the standard deviation (8.90 us) is 43% of the mean (20.6 us)
* the minimum (8.50 us) is 59% smaller than the mean (20.6 us)
* the maximum (41.5 us) is 102% greater than the mean (20.6 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

gevent spawn: Mean +- std dev: 20.6 us +- 8.9 us
.........
gevent sleep: Mean +- std dev: 4.20 us +- 0.21 us
.........
WARNING: the benchmark result may be unstable
* the standard deviation (8.52 us) is 50% of the mean (17.2 us)
* the minimum (7.74 us) is 55% smaller than the mean (17.2 us)
* the maximum (58.1 us) is 238% greater than the mean (17.2 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn: Mean +- std dev: 17.2 us +- 8.5 us
.........
WARNING: the benchmark result may be unstable
* the standard deviation (968 ns) is 18% of the mean (5.26 us)
* the maximum (10.5 us) is 99% greater than the mean (5.26 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool sleep: Mean +- std dev: 5.26 us +- 0.97 us
..........
WARNING: the benchmark result may be unstable
* the standard deviation (1.45 us) is 52% of the mean (2.80 us)
* the maximum (5.50 us) is 96% greater than the mean (2.80 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventraw spawn: Mean +- std dev: 2.80 us +- 1.45 us
.........
geventraw sleep: Mean +- std dev: 4.24 us +- 0.27 us
.........
none spawn: Mean +- std dev: 1.10 us +- 0.06 us

I get this on 3.6.4:

.....................
eventlet spawn: Mean +- std dev: 12.2 us +- 1.2 us
.....................
eventlet sleep: Mean +- std dev: 16.3 us +- 0.8 us
.....................
gevent spawn: Mean +- std dev: 13.1 us +- 1.1 us
.....................
gevent sleep: Mean +- std dev: 10.4 us +- 0.8 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (2.73 us) is 17% of the mean (16.1 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn: Mean +- std dev: 16.1 us +- 2.7 us
.....................
geventpool sleep: Mean +- std dev: 11.2 us +- 0.5 us
.....................
geventraw spawn: Mean +- std dev: 4.95 us +- 0.42 us
.....................
geventraw sleep: Mean +- std dev: 7.34 us +- 0.28 us
.....................
none spawn: Mean +- std dev: 1.98 us +- 0.05 us
.....................
geventpool join: Mean +- std dev: 6.59 us +- 0.25 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (1.28 us) is 10% of the mean (12.7 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

eventlet spawn kwarg: Mean +- std dev: 12.7 us +- 1.3 us
.....................
gevent spawn kwarg: Mean +- std dev: 14.6 us +- 1.2 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (2.81 us) is 17% of the mean (17.0 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn kwarg: Mean +- std dev: 17.0 us +- 2.8 us
.....................
geventraw spawn kwarg: Mean +- std dev: 6.11 us +- 0.45 us
.....................
none spawn kwarg: Mean +- std dev: 2.22 us +- 0.07 us

And this on 2.7.14:

.....................
WARNING: the benchmark result may be unstable
* the standard deviation (2.10 us) is 11% of the mean (18.4 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

eventlet spawn: Mean +- std dev: 18.4 us +- 2.1 us
.....................
eventlet sleep: Mean +- std dev: 23.1 us +- 0.8 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (4.39 us) is 25% of the mean (17.3 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

gevent spawn: Mean +- std dev: 17.3 us +- 4.4 us
.....................
gevent sleep: Mean +- std dev: 10.3 us +- 0.5 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (3.92 us) is 16% of the mean (24.7 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn: Mean +- std dev: 24.7 us +- 3.9 us
.....................
geventpool sleep: Mean +- std dev: 13.5 us +- 0.9 us
.....................
geventraw spawn: Mean +- std dev: 6.91 us +- 0.49 us
.....................
geventraw sleep: Mean +- std dev: 8.95 us +- 0.30 us
.....................
none spawn: Mean +- std dev: 2.21 us +- 0.04 us
.....................
geventpool join: Mean +- std dev: 7.93 us +- 0.28 us
.....................
eventlet spawn kwarg: Mean +- std dev: 17.4 us +- 1.3 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (4.11 us) is 27% of the mean (15.1 us)
* the maximum (24.2 us) is 60% greater than the mean (15.1 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

gevent spawn kwarg: Mean +- std dev: 15.1 us +- 4.1 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (4.74 us) is 18% of the mean (26.8 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn kwarg: Mean +- std dev: 26.8 us +- 4.7 us
.....................
WARNING: the benchmark result may be unstable
* the standard deviation (959 ns) is 12% of the mean (8.00 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventraw spawn kwarg: Mean +- std dev: 8.00 us +- 0.96 us
.....................
none spawn kwarg: Mean +- std dev: 2.48 us +- 0.06 us

Partial PyPy results:

.........
WARNING: the benchmark result may be unstable
* the standard deviation (5.77 us) is 18% of the mean (32.5 us)
* the maximum (52.2 us) is 61% greater than the mean (32.5 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

eventlet spawn: Mean +- std dev: 32.5 us +- 5.8 us
.........
eventlet sleep: Mean +- std dev: 39.9 us +- 2.4 us
.........
WARNING: the benchmark result may be unstable
* the standard deviation (8.90 us) is 43% of the mean (20.6 us)
* the minimum (8.50 us) is 59% smaller than the mean (20.6 us)
* the maximum (41.5 us) is 102% greater than the mean (20.6 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

gevent spawn: Mean +- std dev: 20.6 us +- 8.9 us
.........
gevent sleep: Mean +- std dev: 4.20 us +- 0.21 us
.........
WARNING: the benchmark result may be unstable
* the standard deviation (8.52 us) is 50% of the mean (17.2 us)
* the minimum (7.74 us) is 55% smaller than the mean (17.2 us)
* the maximum (58.1 us) is 238% greater than the mean (17.2 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool spawn: Mean +- std dev: 17.2 us +- 8.5 us
.........
WARNING: the benchmark result may be unstable
* the standard deviation (968 ns) is 18% of the mean (5.26 us)
* the maximum (10.5 us) is 99% greater than the mean (5.26 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventpool sleep: Mean +- std dev: 5.26 us +- 0.97 us
..........
WARNING: the benchmark result may be unstable
* the standard deviation (1.45 us) is 52% of the mean (2.80 us)
* the maximum (5.50 us) is 96% greater than the mean (2.80 us)

Try to rerun the benchmark with more runs, values and/or loops.
Run 'python -m perf system tune' command to reduce the system jitter.
Use perf stats, perf dump and perf hist to analyze results.
Use --quiet option to hide these warnings.

geventraw spawn: Mean +- std dev: 2.80 us +- 1.45 us
.........
geventraw sleep: Mean +- std dev: 4.24 us +- 0.27 us
.........
none spawn: Mean +- std dev: 1.10 us +- 0.06 us

Honor PURE_PYTHON at runtime

Fixes #1118.

Also some speed improvements for semaphores.

Speed up gevent.local.local more, especially for subclasses (2-3x faster)

Do this with some cython tricks and some caching of type attributes.
Cython 0.28 compiles this code to be quite a bit faster than Cython
0.27 does (the below uses 0.28), but it's still a win on both.

The type caching could potentially be a compatibility issue, but in
practice I suspect it won't be.

Benchmarks on 3.6:

+--------------------+------------------+-----------------------------+
| Benchmark          | 3.6              | 3.6 This Branch             |
+====================+==================+=============================+
| getattr gevent     | 190 ns           | 158 ns: 1.20x faster (-17%) |
+--------------------+------------------+-----------------------------+
| setattr gevent     | 180 ns           | 165 ns: 1.09x faster (-8%)  |
+--------------------+------------------+-----------------------------+
| getattr gevent sub | 540 ns           | 175 ns: 3.09x faster (-68%) |
+--------------------+------------------+-----------------------------+
| setattr gevent sub | 528 ns           | 179 ns: 2.95x faster (-66%) |
+--------------------+------------------+-----------------------------+
| setattr native     | 80.8 ns          | 78.8 ns: 1.03x faster (-2%) |
+--------------------+------------------+-----------------------------+

Not significant (3): getattr native; getattr native sub; setattr native sub

Benchmarks on 2.7:

+--------------------+------------------+-----------------------------+
| Benchmark          | local_27_master2 | local_27_tweak2             |
+====================+==================+=============================+
| getattr gevent     | 162 ns           | 158 ns: 1.03x faster (-3%)  |
+--------------------+------------------+-----------------------------+
| setattr gevent     | 173 ns           | 165 ns: 1.04x faster (-4%)  |
+--------------------+------------------+-----------------------------+
| getattr gevent sub | 471 ns           | 181 ns: 2.61x faster (-62%) |
+--------------------+------------------+-----------------------------+
| setattr gevent sub | 462 ns           | 190 ns: 2.44x faster (-59%) |
+--------------------+------------------+-----------------------------+
| getattr native     | 87.4 ns          | 89.7 ns: 1.03x slower (+3%) |
+--------------------+------------------+-----------------------------+
| setattr native     | 133 ns           | 110 ns: 1.20x faster (-17%) |
+--------------------+------------------+-----------------------------+
| getattr native sub | 101 ns           | 97.9 ns: 1.03x faster (-3%) |
+--------------------+------------------+-----------------------------+
| setattr native sub | 118 ns           | 111 ns: 1.06x faster (-6%)  |
+--------------------+------------------+-----------------------------+

PyPy unfortunately shows as a little slower, though I'm not sure I
fully believe that:

+--------------------+-------------------+-----------------------------+
| Benchmark          | local_pypy_master | local_pypy_tweak2           |
+====================+===================+=============================+
| getattr gevent     | 135 ns            | 157 ns: 1.16x slower (+16%) |
+--------------------+-------------------+-----------------------------+
| setattr gevent     | 126 ns            | 162 ns: 1.28x slower (+28%) |
+--------------------+-------------------+-----------------------------+
| getattr gevent sub | 150 ns            | 175 ns: 1.17x slower (+17%) |
+--------------------+-------------------+-----------------------------+
| setattr gevent sub | 153 ns            | 183 ns: 1.20x slower (+20%) |
+--------------------+-------------------+-----------------------------+
| getattr native     | 0.18 ns           | 0.19 ns: 1.05x slower (+5%) |
+--------------------+-------------------+-----------------------------+
| setattr native     | 0.18 ns           | 0.20 ns: 1.06x slower (+6%) |
+--------------------+-------------------+-----------------------------+
| getattr native sub | 0.18 ns           | 0.19 ns: 1.05x slower (+5%) |
+--------------------+-------------------+-----------------------------+
| setattr native sub | 0.19 ns           | 0.18 ns: 1.06x faster (-6%) |
+--------------------+-------------------+-----------------------------+

[skip ci]
Current numbers:

local_36_master
===============

getattr gevent: Mean +- std dev: 216 ns +- 6 ns
setattr gevent: Mean +- std dev: 215 ns +- 6 ns
getattr gevent sub: Mean +- std dev: 600 ns +- 19 ns
setattr gevent sub: Mean +- std dev: 591 ns +- 38 ns
getattr native: Mean +- std dev: 56.9 ns +- 4.1 ns
setattr native: Mean +- std dev: 84.3 ns +- 4.9 ns
getattr native sub: Mean +- std dev: 65.6 ns +- 5.3 ns
setattr native sub: Mean +- std dev: 76.2 ns +- 4.6 ns

local_27_master
===============

getattr gevent: Mean +- std dev: 177 ns +- 7 ns
setattr gevent: Mean +- std dev: 192 ns +- 6 ns
getattr gevent sub: Mean +- std dev: 501 ns +- 10 ns
setattr gevent sub: Mean +- std dev: 536 ns +- 25 ns
getattr native: Mean +- std dev: 93.8 ns +- 3.6 ns
setattr native: Mean +- std dev: 118 ns +- 6 ns
getattr native sub: Mean +- std dev: 102 ns +- 4 ns
setattr native sub: Mean +- std dev: 118 ns +- 8 ns

local_pypy_master
=================

getattr gevent: Mean +- std dev: 135 ns +- 6 ns
setattr gevent: Mean +- std dev: 126 ns +- 4 ns
getattr gevent sub: Mean +- std dev: 150 ns +- 4 ns
setattr gevent sub: Mean +- std dev: 153 ns +- 7 ns
getattr native: Mean +- std dev: 0.18 ns +- 0.00 ns
setattr native: Mean +- std dev: 0.18 ns +- 0.01 ns
getattr native sub: Mean +- std dev: 0.18 ns +- 0.01 ns
setattr native sub: Mean +- std dev: 0.19 ns +- 0.01 ns

Add Greenlet.minimal_ident, a small, sequential, reusable counter for greenlets

Fixes #755

Add spawn_tree_locals, spawning_greenlet and spawning_stack to gevent.greenlet.Greenlet