objgenerator: Handle default args to generator functions.

This addresses initial report in #397.

Despite what's written in comments to new function, I now think that the forward is to make even mp_execute_byte_code() accept pointer to state array. We allocate that unconditionally, so should use it right away for argument munging, instead of allocating temporary buffers. for them to be eventually copied into state.

(But please merge this nonetheless, otherwise it's "lost in refactoring" - besides, I'm trying to make test_pep380.py run as far as possible).