Add robin_hood::unordered_hash_map, from https://github.com/martinus/robin-hood-hashing/
commit 8eb44ddef7118f1045781895108cdd4d6cbbd0e2. We only include the header file
(unmodified), not the tests, build system etc.

Change-Id: I8fecc88216b453dff8f47041d61b81d0eafdd936