* Optimise clearing by adding `nullptr` overload. This overload means
  `Callback(NULL)` or `Callback(0)` will no longer work; users must
  use `Callback(nullptr)` or `Callback()`.
* Optimise clearing by not clearing storage - increases code size of
  comparison, but that is extremely rare.
* Reduce ROM used by trivial functors - share copy/destroy code.
* Config option to force trivial functors - major ROM saving by
  eliminating the "operations" table.
* Config option to eliminate comparison altogether - minor ROM saving by
  eliminating zero padding.
* Conform more to `std::function` API.