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 see faster-cpython/ideas#457

To do this we need:

• Shim frames to keep the various returns simple GH-96421: Insert shim frame on entry to interpreter #96319
• Modify FOR_ITER to leave iterator and value on the stack when exiting GH-96793: Change FOR_ITER to not pop the iterator on exhaustion. #96801
• Implement specialization of FOR_ITER for generators GH-96793: Specialize FOR_ITER for generators. #98772
• Implement specialization of SEND for coroutines

Edited:
Remove the requirement for a GEN_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