Suggestion: add modify_, change modify.

[artemisSystem]

Currently, we have modify :: forall s r. (s -> s) -> Run (state :: STATE s | r) Unit.
It would be nice to instead have:
modify :: forall s r. (s -> s) -> Run (state :: STATE s | r) s (returns the new state)
modify_ :: forall s r. (s -> s) -> Run (state :: STATE s | r) Unit (current modify)

This would be a breaking change, but it could be added along with all the changes that are necesscary for 0.14, since they will be breaking as well.