Add a suffix(while:) method

[natecook1000]

The standard library includes prefix and suffix methods that take a number of elements or an index, but only a prefix method that takes a predicate. We should add a suffix(while:) method to match prefix(while:) for bidirectional collections.

[michiboo]

Hi I would like to take this issue! where should I add suffix(while:) methods?

[kylemacomber]

Please open a PR against this repo. It probably makes sense to add a new file, say Sources/Algorithms/Suffix.swift, and add your implementation there.