StatTimer comparisons

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.