Rewrite HashMapHasherDefault based on type traits - it is now possible to declare a default hashing function for any type.

Remove cross-project includes from `hashfuncs.h`.
Improve hashing function for `Color` (based on values instead of `String`).
Move `Variant` comparison from `hash_map.h` to `dictionary.cpp` (`VariantComparatorDictionary`), where it's used.
Remove now unnecessary `HashableHasher`.

core/string/ustring.h

@@ -34,6 +34,7 @@
 
 #include "core/string/char_utils.h" // IWYU pragma: export
 #include "core/templates/cowdata.h"
+#include "core/templates/hashfuncs.h"
 #include "core/templates/vector.h"
 #include "core/typedefs.h"
 #include "core/variant/array.h"
@@ -232,6 +233,8 @@ public:
 		return *this;
 	}
 
+	uint32_t hash() const { return hash_djb2(get_data()); }
+
 protected:
 	void copy_from(const T *p_cstr) {
 		if (!p_cstr) {