per-interpreter: pending calls (from ceval state)

[ericsnowcurrently]

(Note: this was reverted, so the underlying problem needs to be addressed and then the PR re-merged.)

Move PyRuntimeState.ceval.pending to PyInterpreterState.ceval.pending.

• move pending calls from PyRuntimeState to PyInterpreterState
• move eval "breaker" flag to PyInterpreterState
• pass PyInterpreterState * to related macros (rather than rely on PyThreadState_Get())
• sort out what "main" thread means, when per-interpreter
• fix usage of Py_AddPendingCall()

[ericsnowcurrently]

https://bugs.python.org/issue35423
python/cpython#10972

https://bugs.python.org/issue33608
python/cpython#11617

others...

[phmc]

Just pinging here too: I've added some thoughts on the crash exposed by the pending calls changes on the bpo issue https://bugs.python.org/issue33608

[ericsnowcurrently]

Thanks! I'll take a look and respond over there. (Hopefully I can get to it today.)