Return an opaque token from `task_status.started()`

[A5rocks]

We can get typecheckers to lint that the function passed to nursery.start calls task_status.started() at least once. Unfortunately exactly once isn't possible, but I think something like:

• add a return value to .started()
• require all functions passed into nursery.start return that return value

and that should work well enough? Inspired by conversation on Gitter, maybe this has already been proposed.