Make the tester blow up if CPython exits with a negative exit code, which indicates some non-Python-level problem

Makes it qucker to run; fixes #20.

Also, set -Wno-extern-c-compat to silence libunwind-related warnings.
Fixes #27, hopefully for real this time.

This doesn't surface if you install libunwind globally since clang will
ignore errors from system libraries; this change should fix it for people
who are installing libunwind locally (as is the goal).
Fixes #27

In particular, lower all control-flow-involving expressions (such as boolops,
list comprehensions, if expressions) into actual control flow.  This requires
unfolding the AST into a flat structure, since the control-flow-expression can
be nested inside other expressions, including other control-flow-expressions.

The IRGenerator was doing this before, but was leading to duplication because
it's very similar to what the CFG generator already has to do.

Doing this earlier in the pipeline means that more analysis passes can work on
the lowered expressions, rather than having to be taught how they work.  In
particular, the name analysis does not need to have to special case the fact
that list comprehensions may set names -- but aren't guaranteed to if they don't
get executed or the iterator is empty.  I tried implementing list comprehensions
without doing this, and a bunch of the work was in reimplementing the logic that
the analyzers already implement.

As a side benefit, deopts become much easier since the AST is already unfolded;
this change gets rid of the messy fake-variable-setting way that the IRGenerator
was unfolding the AST to get this same effect.

fix text formatting 

Update README.md

List ccache as required in INSTALLING

Should fix #22.

Many args ir fix

This shows up for both pull requests and creating new issues; make this more clear that the CLA only applies to pull requests if you come from the issue page.

It looks like the culprit is IRBuilder<>

separate the irgenerator into another file; unfortunately it doesn't seem to improve compilation times at all

Update README.md

return statement added to python example.