Issue Description:

In Swift 3.0, StrideTo and StrideThrough conform to Sequence, but not Collection.

While they can't be bi-directional or random-access, as the last (or abitrary) element can't necessarily be reached in O( n ) for all strideable types, it should be possible to make them forward-only collections. One straightforward implementation would be to make the index each value zipped with a count (for Comparable purposes).

Once we have conditional conformance, it should be possible to make specific strides (e.g. ones over integers) conform to random access.