- change up interpreter-shutdown to better deal with objects with custom finalizers
- fix a bunch of mixed decrefs for oldstyle-classes
- a couple other misc things

file.py is now working

I guess a number of tests do this to help with testing

instead of re-calculating it for every BB

profiling ftw

Don't use ValueMap::operator[], since that is much more
expensive than lookup().

I think `make check_dbg` is clean again.

Most of these need to return borrowed references in order to match the C API.

If we find a type that
- we think we can reason about statically
- does not have an __iter__ but can be iterated via __getitem__

we previously just bailed saying that we know it doesn't have an __iter__ method
(instead of calling getiterHelper like we should).  This bug exists on master as well,
but we just don't run into it that often (especially since it's llvm-tier-only).

With some prefetching.

Now that they are only a small number of sizes (powers of two),
can use a freelist pretty easily.  Especially because most of the
allocations will come from places where we can precompute the freelist
index.

About a 5% improvement on raytrace.py

Our old allocator supported very fast realloc() calls (amortized constant
time), and we relied on that by doing "realloc(array, old_size + 1)" every
time we wanted to grow the array.

Instead, now do "if (power_of_two(old_size)) realloc(array, old_size * 2)"
Somehow this is 15% faster overall.

Refcounting added a second way to represent "no attrs" -- NULL as a hcls.
This was causing guards to fail since a check would be written for one
but then it would fail when objects came in with the other.

So canonicalize to using NULL as the "no attributes" hcls.