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/ref_counted.h

@@ -216,6 +216,8 @@ public:
 		ref(memnew(T(p_params...)));
 	}
 
+	uint32_t hash() const { return HashMapHasherDefault::hash(reference); }
+
 	Ref() = default;
 
 	~Ref() {