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 62f2298459da0114393b1e37b1f0b50718f17f08 on unwrap_it into eb6e410dec07be94f6d2143a3b0cf6ac32a0ce30 on master.