[flat_map] [flat_set] allow initialization from sorted containers

[iboB]

In certain cases we know that a container is sorted. flat_map and flat_set would benefit if they can be initialized with a sorted container to save us calls to std::unique and std::sort.

This is currently possible by using modify_container as in:

flat_something foo;
foo.modify_container() = std::move(sorted_container);

...but that's not initialization. foo can't be const in such an example.

My thinking is to use a tag like:

flat_map foo = flat_map::sorted_tag(std::move(container))

Is there anything better?