golang: Move channels implementation from golang.py to golang.pyx

Plain code movement with just s/panic/pypanic/ as in golang.pyx panic is
aleady there and means semantically different thing. Moved code, even
though it lives in golang.pyx, is still Python code and requires Python
runtime and GIL. We'll be splitting channels implementation into nogil
world in the following patches.

Just plain movement to Cython brings the following speedup:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               26.6µs ± 1%  21.7µs ± 1%  -18.54%  (p=0.000 n=10+10)
    chan             13.7µs ± 1%   9.9µs ± 4%  -27.80%  (p=0.000 n=10+10)
    select           29.3µs ± 2%  19.2µs ± 4%  -34.65%  (p=0.000 n=9+9)
    def              55.0ns ± 0%  58.0ns ± 0%   +5.45%  (p=0.000 n=10+10)
    func_def         44.0µs ± 1%  44.4µs ± 0%   +0.72%  (p=0.002 n=10+10)
    call             64.0ns ± 0%  63.0ns ± 0%   -1.56%  (p=0.002 n=8+10)
    func_call        1.09µs ± 1%  1.05µs ± 1%   -2.96%  (p=0.000 n=10+10)
    try_finally       139ns ± 2%   135ns ± 0%   -2.60%  (p=0.000 n=10+10)
    defer            2.36µs ± 1%  2.36µs ± 1%     ~     (p=0.617 n=10+10)
    workgroup_empty  58.1µs ± 1%  49.0µs ± 1%  -15.61%  (p=0.000 n=10+10)
    workgroup_raise  72.7µs ± 1%  62.6µs ± 1%  -13.88%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               28.6µs ± 0%  25.4µs ± 0%  -11.20%  (p=0.000 n=8+9)
    chan             15.8µs ± 1%  12.2µs ± 1%  -22.62%  (p=0.000 n=10+10)
    select           33.1µs ± 1%  23.3µs ± 2%  -29.60%  (p=0.000 n=10+10)
    def              55.0ns ± 0%  56.0ns ± 0%   +1.82%  (p=0.000 n=10+10)
    func_def         44.4µs ± 2%  43.0µs ± 1%   -3.06%  (p=0.000 n=10+9)
    call             64.0ns ± 2%  69.0ns ± 0%   +7.81%  (p=0.000 n=10+10)
    func_call        1.06µs ± 0%  1.06µs ± 1%     ~     (p=0.913 n=8+9)
    try_finally       136ns ± 0%   139ns ± 0%   +2.21%  (p=0.000 n=9+10)
    defer            2.29µs ± 1%  2.38µs ± 2%   +3.58%  (p=0.000 n=10+10)
    workgroup_empty  73.8µs ± 1%  70.5µs ± 1%   -4.48%  (p=0.000 n=10+10)
    workgroup_raise  94.1µs ± 0%  90.6µs ± 0%   -3.69%  (p=0.000 n=10+10)

golang/golang_test.py

@@ -30,7 +30,7 @@ from six.moves import range as xrange
 import gc, weakref
 
 import golang
-from golang import _chan_recv, _chan_send
+from golang._golang import _chan_recv, _chan_send
 from golang._pycompat import im_class
 
 # pyx/c/c++ tests -> test_pyx_*
@@ -693,13 +693,14 @@ def bench_select(b):
 
 
 def test_blockforever():
-    B = golang._blockforever
+    from golang import _golang
+    B = _golang._blockforever
     def _(): panic("t: blocks forever")
-    golang._blockforever = _
+    _golang._blockforever = _
     try:
         _test_blockforever()
     finally:
-        golang._blockforever = B
+        _golang._blockforever = B
 
 def _test_blockforever():
     z = nilchan