Add constexpr to advance, distance, next, and prev

[ericniebler]

Also common_iterator(?).

[CaseyCarter]

My review notes were more along the lines of "Add constexpr to those things that are constexpr in the WD, notably advance, distance, next, prev." reverse_iterator and move_iterator are also heavily constexpr in the WD; there was a strong consistency argument that common_iterator and counted_iterator should be as well.

[ericniebler]

With the caveat that we have to be careful with common_iterator to not disallow a variant-like implementation.

[CaseyCarter]

Resolved in P0579.