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/object/object.h

@@ -641,7 +641,7 @@ private:
 		};
 
 		MethodInfo user;
-		HashMap<Callable, Slot, HashableHasher<Callable>> slot_map;
+		HashMap<Callable, Slot> slot_map;
 		bool removable = false;
 	};
 	friend struct _ObjectSignalLock;