per-interpreter: gc

[ericsnowcurrently]

Move PyRuntimeState.gc to PyInterpreterState.gc.

Notable concerns:

• can be moved separately from rest of mem state?
• do we still need a global state for anything? (just use "main"?)

(https://bugs.python.org/issue36854)

[vstinner]

I closed https://bugs.python.org/issue36854 so this issue can be closed as well, no?

[ericsnowcurrently]

oh, funny, I moved it to "done" but that didn't close it :)