Add a PYSTON_NOEXCEPT define that gets defined to "noexcept" in C++ mode,
and to the empty string in C mode.

I don't think 'extern "C"' implies noexcept.

This is partly for better performance when we know that a function cannot throw
an exception, but also as an annotation for us since the exception model is
the main difference between C land and Pyston land.

Vim substitution: %s/\(\<PyAPI_FUNC\>(.*).*(\([^)]\|\n\)*)\);/\1 PYSTON_NOEXCEPT;/gc
- This will catch almost all cases, except for functions not marked with PyAPI_FUNC
  and function definitions that have extra paretheses (in comments, usually)