and add a pyc "stress test".

I think these issues are the source of our sporadic ci failures;
it makes sense based on where things fail (usually in the parser), and
because it's stateful (if you already have pycs generated you don't
run into the issue) and because it only happens in multithreaded mode.

changes:
- read the entire file at once, then do checks
- add a simple xor checksum in addition to the expected length

Add pyexpat support

Some minor merge conflicts due to duplication with #544.

Faster version of attrwrapper.clear()

implement long.__pos__

Add a bunch of capi and fix a bug inside PyNumber_Int (was throwing a c++ exception)

It's no longer being used, and isn't safe to use any more
since it's not invalidated the times it needs to be.

Misc stuff

Generators: Support explicit StopIteration raising.

Before this patch we couldn't run 'list(G())' if G contained a explicit raise StopIteration statement.
This also adds support for custom StopIteration subclasses.

(was throwing a c++ exception)

unwinder

The signal-blocking is used to prevent libunwind from being entered
recursively, if a signal handler invokes libunwind. We don't call
libunwind from signal handlers, so this is not a problem for us.

The conservative checks just make crashes behave more nicely. They have
a small but measurable performance impact. We enable them in debug mode
and disable them for release mode.

We also modify .gitmodules so that our patches to libunwind/ don't make
git think the tree is dirty.

http://blog.jgc.org/2015/04/the-one-line-you-should-add-to-every.html

Perf investigations

The issue is that sometimes we want to test out using builds
that wouldn't be able to self-host the libraries that the build
system thinks we need.  So, add a flag to just not build those.

I think this only matters if we knew the type at JIT time.

also, improve the annotator