maitake: add JoinHandle

[hawkw]

we should have that.

[hawkw]

essentially a JoinHandle is a wrapper type around a TaskRef that impls Future. in the simplest impl, you would have a WaitCell in the task struct for the join-interested task's waker, and if that's set when the task completes, you notify that task. the task vtable would add a function for moving the output out of the task cell, and the JoinHandle future would call that after it's woken.

eventually, we could use the existing state atomic for the join waker state instead of using a WaitCell that adds another word when it really just needs a couple bits. this would be more efficient, but WaitCell is probably good to start with.

[hawkw]

since TaskRefs are ref counted anyway, the JoinHandle would already keep the task alive while it exists (even after completion), and ensure it's deallocated on drop if the handle is the last ref.

[hawkw]

our JoinHandles don't have to worry about capturing a task panic like tokio/async-std's do, because maitake currently assumes panics are fatal (and neither of our operating systems support unwinding :P)

we might still want to return a Result if we want to add remote task aborts, though...

[hawkw]

@jamesmunns this could probably be pretty self-contained if you want something else to hack on...