use copy of initial state machine to get resumption back to zero

[dsyme]

No description provided.

[abelbraaksma]

Thanks for these improvements, @dsyme, and thanks for pairing with me to fix this gnarly issue! For posterity, the fix presented here does two things:

• it creates a shadow copy of the original state machine just after creating of the state machine. It then uses this shadowed machine as the default state for the cloned enumerator (needed as soon as there is more than one call to GetAsyncEnumerator()). This replaces the Unchecked.defaultof hack I used in Fix resumability of iteration over IAsyncEnumerable<'T> #42, which only partially solved the issue, and created an nondeterministic state.
• it removes some allocations and unnecessary logic in the GetAsyncEnumerator(), among other things, this removes the necessity for MemberwiseClone.