objgenerator: Another obscure case of propagating MP_OBJ_NULL optimization.
objgenerator.throw(GeneratorExit) is not equivalent to .close().

.throw() propagates any exceptions, and .close() swallows them. Yielding
in reponse to .throw(GeneratorExit) is still fatal, and we need to
handle it for .throw() case separately (previously it was handled only
for .close() case).

Obscure corner cases due to test_pep380.py.
mp_resume: Elaborate handling of .throw() for objects which lack it.

In this case, the exception is just re-thrown - the ideas is that object
doesn't handle this exception specially, so it will propagated per Python
semantics.
mp_resume: Dare to pass send_value of NULL.

There was thinkos that either send_value or throw_value is specified, but
there were cases with both. Note that send_value is pushed onto generator's
stack - but that's probably only good, because if we throw exception into
gen, it should not ever use send_value, and that will be just extra "assert".
objtype: Add virtual __name__ attribute.

It's virtual because it's not shown in dir(...). (That's also how CPython
has it).