Support multiple iterators in for loop assign clauses

[robertkleffner]

Currently only one is supported due to compilation trickiness, would like to resolve this and support more than one.

Since iterator for loops are compiled down to handlers maybe there is a straightforward way to do this with injection.

Or maybe a dotted effect? That would be interesting, semantics could probably handle it but the type system might not.

Worst case scenario it can be done with a few definitions for multiple argument iterators, i.e. yield1!, yield2!, etc.