bpo-36710: Add PyInterpreterState.runtime field (GH-17270)

Add PyInterpreterState.runtime field: reference to the _PyRuntime
global variable. This field exists to not have to pass runtime in
addition to tstate to a function.  Get runtime from tstate:
tstate->interp->runtime.

Remove "_PyRuntimeState *runtime" parameter from functions already
taking a "PyThreadState *tstate" parameter.

_PyGC_Init() first parameter becomes "PyThreadState *tstate".