Optimize Iterators

[ochafik]

Iterator sources / .toIterator ops should be easy to fit in as long as the final sink is not an Iterator.
(otherwise the source emission logic must be refined)

[ochafik]

Iterator sources are now supported (List(1).toIterator.map(a).filter(b).{sum, toList} works).
Supporting sinks (i.e. Iterator(1).map(a)) is an interesting subject, will need some thought.

Also TODO is to make Strategies aware of iterators: all the side-effect analysis is needless for iterators (it's precisely there to prevent accidental change of semantic between materialized collections and iterators), but it depends where iterators are in the stream.

For instance: list.map(a).map(b).toIterator.map(c).map(d).toList.map(e). Can't fuse the stream with any safe strategy if a and b both have side-effects, but c and d are okay, etc...