Adding C API for use in tp_traverse

[emmatyping (Emma Smith)]

@ngoldbaum

@encukou

This currently can’t happen: both GC and MRO changes are stop-the-world operations. But that doesn’t seem like something we want to guarantee in the long term.

Maybe the stop-the-world part is the key aspect and the part that should be highlighted in the name?

As @encukou says at the end there, we probably don’t want to guarantee stop-the-world in the long-term as it’s really an implementation detail that could change. I’d like to keep the door open for a different design for the GC that may not be stop-the-world.