Make state_transition not return post-state

[hwwhww]

Issue

1. It's arguable that the mutability side effect is smell (see Side effects in functions #1932 discussion). Although I personally want to remove the side effect, we decide to keep it for now for the code concision and stability.
2. What worse than the mutability side effect is that the state_transition provides two patterns: (i) it mutates the given state and also (ii) returns the post-state. We're using (i) in the whole specs and (ii) only exists in state_transition.

How did I fix it

Make state_transition not return post-state. The given pre-state should have been mutated to post-state.

[djrtwo]

taking back my approval. going to make sure this isn't used in any unexpected way in tests/test-gen