missing feature: task- and run-local storage

[njsmith]

Similar to what I implemented in curio

It'd be nice if run_in_worker_thread preserved the task-local context.

[njsmith]

While we're at it we should probably have "run local" storage too: a place to stash stuff that's local to this run, and which automatically disappears when the run ends. Example: the getaddrinfo semaphore (#10).

Technically this is possible to fake already by using thread-local storage + a system task for notification of the run shutdown, but there's no reason to make this that difficult.

[njsmith]

Then again, it's also possible to fake run-local storage by just setting a task-local storage key early on (like in your main task) and then letting it be inherited by all child tasks... though this does waste some memory if you have lots of tasks.