* 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.
Kevin Bracey