Low level review sent.

Regarding the new opcode, it appears the main thing it provides is early detection of yielding from a coroutine in a normal generator, which is never going to work properly (a for loop or other iterative construct can't drive a coroutine as it expects to be driven), but would be incredibly painful to debug if you did it accidentally.

For me, that counts as a good enough reason to add a new opcode during the beta period (just as adding the new types is necessary to fix various problems with the original not-actually-a-different-type implementation).