[capi-sig]Adding an official (minimal) Cython-like tool.

Not that long ago Brett, Barry, and I were talking about how to get extension authors to move away from the C-API. Cython is the obvious choice, but it isn't an official tool nor does it necessarily make sense to make it one. Regardless, it would help all parties involved if there *were* an official tool that was part of CPython (i.e. in the repo). Cython could build on top of it and extension authors would be encouraged to use it or Cython. If such a thing makes sense, I figure we would follow the pattern set by asyncio (relative to twisted).

-eric