We want to be able to specialize iteration over generators and awaiting coroutines. Both for PEP 659 and for higher tier optimizers.

For the theory seefaster-cpython/ideas#457

To do this we need:

• Shim frames to keep the various returns simpleGH-96421: Insert shim frame on entry to interpreter #96319
• ModifyFOR_ITER to leave iterator and value on the stack when exitingGH-96793: ChangeFOR_ITER to not pop the iterator on exhaustion. #96801
• Implement specialization ofFOR_ITER for generatorsGH-96793: Specialize FOR_ITER for generators. #98772
• Implement specialization ofSEND for coroutines

Edited:
Remove the requirement for aGEN_RETURN_VALUE instruction. We might want one to simplify frame popping, but it isn't needed for specializing.

• PR:GH-96793: Implement PEP 479 in bytecode. #99006

• PR:GH-96793: Specialize FOR_ITER for generators. #98772