MoveNextAsync should guard against out-of-order invocation

[jcouv]

If a promise is already pending, the caller of the state machine should not be calling MoveNextAsync().

[jcouv]

Per discussion with LDM, we'll only be guarding calls to DisposeAsync at the wrong time. The spec will call out unspecified behavior when mis-using the APIs, but we will attempt to guard all of them.