Pass value object attributes by reference (instead of pointer)

This underlines that no reference inside the setter is taken,
and that these attributes are non-nullable.

Also removes a couple of superfluous attribute macros.

binding-mruby/binding-util.h

@@ -166,7 +166,7 @@ defineClass(mrb_state *mrb, const char *name)
 		PropKlass *prop; \
 		mrb_get_args(mrb, "o", &propObj); \
 		prop = getPrivateDataCheck<PropKlass>(mrb, propObj, PropKlass##Type); \
-		GUARD_EXC( k->set##PropName(prop); ) \
+		GUARD_EXC( k->set##PropName(*prop); ) \
 		return propObj; \
 	}