Add remote spawns

[NthTensor]

This adds a new spawn_on operator, that sends work to a remote thread using the same underlying queue as broadcasts. This operator is needed for game-engine scheduling, where we want bias the normal amortization/stealing path to eagerly distribute a small number of long-running tasks.

I also changed the Spawn and SpawnScoped traits to decouple task-type (closure, or async function) from task-scheduling (shared, local, or remote).

Every time I do stuff like this, I have to run a full safety audit to ensure all the arguments still work. Usually I find a bunch of places where I can improve the existing safety proofs. This time was no different; I've made a lot of small improvements to safety arguments across the repo, and fixed a few minor things.