Remove EndState?

[ethanresnick]

Pros: simpler type set. See no-end-state branch.

Cons: makes it impossible to distinguish between an async state that is semantically an "end state" and an async state that isn't (i.e., because its async behavior is going to redirect the call eventually), as both can show up as renderable states with no branching.

[ethanresnick]

Another option: remove isEndState for now, but add it back later as an optional flag on non-branching renderable states in the future iff it becomes necessary/we attach functionality to the semantic idea of an "end state".