Store wakers in a Slab instead of a Vec

[jplatte]

The second commit is the interesting bit, where we drop wakers for subscribers that are no longer waiting for updates due to being dropped... However, we probably also need to catch the case where the subscriber still exists / at least detect waker_key being Some when it is being set.

Note to self: Add tests for both subscribers being dropped early, and subscriber awaiting being timed out.

Closes #42.

[jplatte]

Note to self: drop waker when next / next_ref future is aborted (dropped), not just when subscriber is dropped.