This works the same as flat_set but corresponding to a std::map.

The existing flat_set implementation was factored into a base class called flat_tree with a provision to have a potentially different lookup key type than value type, and a way to convert from a value to a key. flat_set and flat_map can be trivially implemented on top of this API.

The existing flat_set tests are moved to flat-tree and some simple type-specific tests were created for the set and map derivations.

flat_tree no longer copies the comparison object except when required (when returned by value_comp()). But this requires that the comparison functor be const which was not the case before.

Review-Url: https://codereview.chromium.org/2715433007
Cr-Commit-Position: refs/heads/master@{#458796}