Replace map<UniqueString*> with UniqueStringMap

Further profiling of stack unwinding found that std::map was fairly
allocation-heavy. Julian implemented a UniqueStringMap class, which stores a
small fixed array of UniqueString* and simply does linear search on them, which
turns out to be faster than using std::map for our use cases (most usage of
PostfixEvaluator tends to have a small number of keys). UniqueStringMap
overflows to a std::map internally. Patch is originally by Julian Seward.