Improve try_mutate aliasing behaviour.
Improve try_mutate aliasing behaviour.

This fixes try_mutate-in-try_mutate and, for debug builds only,
borrow-in-try_mutate. Fixing borrow-in-try_mutate for release builds is
negotiable, but I think I’d prefer the performance, for something that I
hope no one would ever write in the first place.

Fixes #1.

chris-morgan/mucell