This commit replaces the ue2::unordered_{set,map} types with their STL
versions, with some new hashing utilities in util/hash.h. The new types
ue2_unordered_set<T> and ue2_unordered_map<Key, T> default to using the
ue2_hasher.
The header util/ue2_containers.h has been removed, and the flat_set/map
containers moved to util/flat_containers.h.
Switch over ptr_vector<T> to vector<unique_ptr<T>>. This works around
some issues we were seeing with MSVC builds, where the contents of the
ptr_vector were being destroyed when it was returned.
