Review sent - very nice work on this Yury.

Highlights:

* I concur with Stefan that we should have a full PyCoroutineMethods struct at the C level, with a "tp_as_coroutine" pointer to that replacing the current tp_reserved slot

* I also concur with Stefan about adding a Coroutine ABC

* PyType_FromSpec (and typeslots.h) will need updating once we agree on a slot structure (with my recommendation being "define C level slots for all of the new PEP 492 methods")

* I found CO_COROUTINE/CO_NATIVE_COROUTINE confusing as a reader of the implementation, as they only told me how the objects were defined, rather than telling me why I should care. Based on what I gleaned of their intended purpose from reading the implementation, I suggest switching this to instead use CO_COROUTINE (set for all coroutines, regardless of how they were defined) and CO_ITERABLE_COROUTINE (set only for those coroutines that also support iteration), and adjusting the naming of other APIs accordingly.

* I found the names of the WITH_CLEANUP_ENTER and WITH_CLEANUP_EXIT bytecodes misleading, as they don't refer to the corresponding context management phases - they're both related to the "exit" phase. WITH_CLEANUP_START and WITH_CLEANUP_FINISH should be clearer for readers (both of the implementation and of the disassembled bytecode).