Using `FunctionRef` is better than using `std::function`, templates and c function
pointers in some cases. The trade offs are explained in more detail in code documentation.
The following are some of the main benefits of using `FunctionRef`:
* It is convenient to use with all kinds of callables.
* It is cheaper to construct, copy and (possibly) call compared to `std::function`.
* Functions taking a `FunctionRef` as parameter don't need to be declared
in header files (as is necessary when using templates usually).
Differential Revision: https://developer.blender.org/D10476
