"foo() -> !" means something different in fns than in proto!

In functions, `-> !` indicates the function returns bottom, i.e., doesn't return. In pipe protocol definitions, `-> !` means "terminate the session", which is analogous to returning unit (the 1 type), not bottom (the 0 type).

It seems more conceptually in-line to write session-end transitions with no arrow, like `... { continue(T) -> state2, close(U) } ...`
