Serialize the dispose of Zip(IEnumerable) with MoveNext/Current

[akarnokd]

IEnumerator.Dispose should not be called while MoveNext or Current is executing as most implementations don't expect a concurrent dispose call I think.

This PR adds a basic trampoline around the calls to MoveNext and Current. This allows a mutual exclusion between them and Dispose while also remembering Dispose should be eventually called:

1. The operator's Dispose is invoked and _enumerationInProgress atomically updates to 1: no concurrent MoveNext is happening and the enumerator can be disposed. If OnNext runs at this point _enumerationInProgress atomically updates to 2 and the method quits without doing anything.
2. OnNext atomically changes _enumerationInProgress to 1 and calls MoveNext. A concurrent Dispose on the operator now updates _enumerationInProgress to 2 and doesn't dispose the enumerator. When MoveNext finishes, it decrements _enumerationInProgress and since this is non-zero, it knows to dispose the enumerator and quit without doing any further processing.

Fixes #132