Backport miscellaneous changes that can't be cherry-picked

(cherry picked from commit 9afbdb9cf6)

include/godot_cpp/core/method_bind.hpp

@@ -412,6 +412,7 @@ public:
 		method = p_method;
 		generate_argument_types(sizeof...(P));
 		set_argument_count(sizeof...(P));
+		set_const(true);
 	}
 };
 
@@ -578,6 +579,7 @@ public:
 		generate_argument_types(sizeof...(P));
 		set_argument_count(sizeof...(P));
 		set_return(true);
+		set_const(true);
 	}
 };