[wip] closable run_sync_soon

[njsmith]

This definitely isn't done, but hey, publish early publish often
right.

The motivation is described in #1085.

Major todo items:

• Figure out what to do with BlockingTrioPortal
• Figure out how to handle TrioToken during the transition
  period. Right now it ignores the handle machinery entirely, which is
  fine until we actually have a deadlock detector.
• Fix tests
• Better name for TrioEntryHandle?

[Fuyukai]

This was never implemented fully and would likely require fixing a whole bunch of merge conflicts. Closing.