… comments

Not currently failing, just demonstrates the unfortunate behavior.

…when due

Also rename `fire_at` to `enqueue_at` to be explicit about the fact.

This is a trade-off that prevents 2 sorts of bad behavior:
- actors with send-sending messages never handling any delayed/recurring messages (#72)
- actors with recurring messages that are slower to handle than their interval eventually not processing any outside messages (#79)

See the tweaked tests for the change of behavior.