The 'internal callable' (bad name, sorry) is what defines how
the arguments get mapped to the parameters, and potentially also does
rewriting.

By providing a custom internal callable, we can make use of special knowledge
about how C API functions work.  In particular, we can skip the allocation
of the args + kwargs objects when we are calling an object with the METH_O
signature.

This patch includes rewriting support, though we don't currently allow
rewriting CAPI functions as part of callattrs.