Support CPython's errnomodule.c, our first extension module!

errnomodule.c is extremely simple and doesn't do anything complicated,
so this is more of a proof-of-concept for now.
The tricky stuff will most likely be around newly-defined types.
Support receiving **kw for builtin functions

Need to special-case some places in the keyword handling
since they previously assumed that they would first have
to check if the keyword name matched a positional argument,
before putting into the output **kw.