Glib::ustring: Add a std::hash<> specialization.
So people can use it as a key for std::unordered_map and std::unordered_set. See #16 (closed), especially #16 (comment 1830387)
@dboles Comments?
@fanc999 Is it correct that std::hash<Glib::ustring> is defined without
GLIBMM_API? The definiton is surrounded by #ifdef GLIBMM_ENABLE_HASH_USTRING_SPECIALIZATION
and that preprocessor macro is not defined when glibmm is built.