VM: Iterator implementation

[timfennis]

In the interpreter the iterator used to have a lifetime of the original value disallowing mutation while in progress. The new VM iterator (in order to quickly fix tests) now usually copies the values it's iterating over which has very different semantics. We should consider the different options we have and what their performance implications are but we probably can't until we can measure performance accurately.

[timfennis]

Performance of our current setup (which mostly doesn't copy) is really good.