Zip2Collection, zip, and enumerated

• Zip2Collection: A Collection-conforming version of Zip2Sequence:

  1. Requires that both bases conform to Collection.
  2. When both bases conform to RandomAccessCollection, Zip2Sequence does also.
  3. When both bases conform to MutableCollection, Zip2Sequence does also.

• func zip<C0, C1>(_ x0: C0, _ x1: C1) -> Zip2Collection<C0, C1> analogous to the Zip2Sequence-returning function in the standard library.

• extension Collection { func enumerated() -> Zip2Sequence<Range<Int>, Element> }, semantically equivalent to the standard library method on Sequence of the same name.