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.

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

Add alloc stats to the base Box::operator new(), which is where non-builtin
types get allocated.

Also add stats for non-Python allocations (typically, conservative memory);
turns out that we are allocating a lot of conservative memory (both in terms
of allocations but also in terms of allocation size) for dicts.

Hopefully this makes it more clear which stats are subsets of others,
and which ones should add up to other ones.

ex it was easy to think that "rewriter_attempts" succeeded as long
as "rewriter_aborts" didn't get incremented, but it could also not
have gotten started, etc.

Here's an example of the new stats:
ic_attempts: 1724625
ic_attempts_nopatch: 1306007
ic_attempts_skipped: 404969
ic_attempts_skipped_megamorphic: 85558
ic_attempts_successful: 13649
ic_rewrites: 13649
ic_rewrites_aborted: 3248
ic_rewrites_aborted_assemblyfail: 65
ic_rewrites_committed: 10401

implement float.hex and float.fromhex

remove the llvm::StringRef from BoxedString

support arg unpacking