Separate initial and finished states for enumerable and async-enumerables

[jcouv]

For enumerables, the problem was introduced recently.
For async-enumerables, the problem existed from the beginning.

Old

• Iterator: N <= -3 (finalize states), -2 (initial for enumerable, finished), -1 (running), 0 (initial for enumerator, ie. start of method), N > 0 (resumable states for 'yield return')
• Async-iterator: N <= -4 (resumable states for yield return) -3 (initial for enumerator) -2 (initial for enumerable, finished) -1 (running) N >= 0 (resumable states for await)
• Async: N <= -3 (vestigial finalizer states in VB) -2 (finished) -1 (running) N >= 0 (resumable states for await)

Goal

Separate the finished and initial states

Changes

• Async-iterator initial state (ie. indicating start of method) is now -4
• Iterator and async-iterator finish on -3
• No changes to async

New

• Iterator: N <= -4 (finalize states), -3 (finished), -2 (initial for enumerable), -1 (running), 0 (initial for enumerator, ie. start of method), N > 0 (resumable states for 'yield return')
• Async-iterator: N <= -5 (resumable states for yield return) -4 (initial for enumerator) -3 (finished) -2 (initial for enumerable) -1 (running) N >= 0 (resumable states for await)

Questions

• I debated changing the finished state for async machines too (from -2 to -3). That would create some alignment across state machine kinds.