More error messages for schwartz_adapter.
More error messages for schwartz_adapter.

std_sorter and std_stable_sorter can't work with schwartz_adapter since
the iter_move/iter_swap customization don't exist yet in the standard
library. This commit introduces readable static assertions to warn user in
case they try to use them together.

Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort