vm: add start_index to iterator unwrap helpers

[ixje]

There's a 2048 max stack item count limit imposed by the VM which prevent our unwrap_iterator helpers from fetching all data from the iterator. By providing a start_index we can work around that limit.

[coveralls]

coverage: 80.497% (-0.1%) from 80.593%
when pulling 62f2298 on unwrap_it
into eb6e410 on master.