Thread Sanitizer Interop

[insertinterestingnamehere]

This adds the API calls needed to help the thread sanitizer correctly carry its happens-before relationships across context swaps.

[insertinterestingnamehere]

Okay, doing another round of careful review of the fences in FEBs seems to have fixed the bugs I was trying to address here. On the other hand, this patch is still causing some mysterious issues. The interface I'm using is poorly documented so it's hard to say for sure if I'm using it correctly. I'm going to leave this open for a bit in case I need to come back to it, but for now I'm going to try to just move on without it.

[insertinterestingnamehere]

Rebased to depend on #256 to avoid conflicts.

[insertinterestingnamehere]

Okay, I think this is at least nominally working. There's something wrong where if we free the tsan fiber objects at the end it segfaults which is bizarre. Maybe something's getting double-freed during runtime cleanup.

[insertinterestingnamehere]

Nevermind, that was just a testing failure causing a false success. Marking this as a draft again for more debugging.

[insertinterestingnamehere]

Okay, this still needs some restructuring for clarity, but it is actually working now.

[insertinterestingnamehere]

Good enough. There's more to do to get thread sanitizer working correctly with our threadqueues, but this is a solid step forward.