Refactor Functors into their own file
Add a new BitFunc abstract type to group Functors that behave in the same bit-wise manner when applied to individual bits as well as integers, allowing them to be used on the BitArray chunks.
Add methods for Functor specialization-by-function value. When no Functor matches, an UnspecializedFunc is returned that simply wraps the function. This should make dispatch simpler when using functors:
```
foo(f::Callable, ...) = foo(specialized_(f), ...)
function foo(f, ...)
# actual implementation here
end
```
