[2018-06 LWG Motion 21] P0879R0 Constexpr for swap and swap related functions

[jensmaurer]

Also resolves LWG issue 2800.

[jensmaurer]

The infrastructure for feature-testing macros arrives with P0941R2, concurrently with applying this paper.
The entry __cpp_lib_constexpr_swap_algorithms must be added.