This PR introduces `Lock`: A concurrency primitive built on top of `Semaphore` that provides a `Mutex`-like primitive that interacts nicely with futures. Specifically, `LockGuard` (in contrast to `MutexGuard`) does _not_ borrow the `Lock`, and can thus be passed into a future where it will later be unlocked.
This replaces #958, which attempted to introduce a less generic version. The primitive proposed there will instead live in [`async-lease`](https://github.com/jonhoo/async-lease).
