Make move assignment non-`virtual`, take the parameter by `&&`, and return by non-`const&`

It is simple and efficient.

See: The rule for copy-assignment.

Equivalent to what is done for copy-assignment.

(Simple) An assignment operator should not be virtual. Here be dragons!
(Simple) An assignment operator should return T& to enable chaining, not alternatives like const T& which interfere with composability and putting objects in containers.
(Moderate) A move assignment operator should (implicitly or explicitly) invoke all base and member move assignment operators.