Order of magnitude faster `map` and `filter`

Speeds up signal's map and filter operations by about an order of magnitude (~7x in my tests, which were not thorough).

Remaining to be done:
- unit testing SlimSignal and SlimSubscriber
- confirmation that the test I made fail and removed was wrong
- confirmation that the different semantics (e.g. no extra layer of locking) match the desired behavior
- confirmation of the speedup