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;

core/object/object_id.h

@@ -30,6 +30,7 @@
 
 #pragma once
 
+#include "core/templates/hashfuncs.h"
 #include "core/typedefs.h"
 
 // Class to store an object ID (int64)
@@ -54,6 +55,8 @@ public:
 	_ALWAYS_INLINE_ void operator=(int64_t p_int64) { id = p_int64; }
 	_ALWAYS_INLINE_ void operator=(uint64_t p_uint64) { id = p_uint64; }
 
+	uint32_t hash() const { return HashMapHasherDefault::hash(id); }
+
 	_ALWAYS_INLINE_ ObjectID() {}
 	_ALWAYS_INLINE_ explicit ObjectID(const uint64_t p_id) { id = p_id; }
 	_ALWAYS_INLINE_ explicit ObjectID(const int64_t p_id) { id = p_id; }

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() {