Introduce a new replace!(new::Callable, res::T, A::T, count::Union{Nothing,Int}) method
which custom types can implement to support all replace and replace! methods automatically,
instead of the current replace!(new::Callable, A::T, count::Int). This offers several advantages:
- For arrays, instead of copying the input and then replace elements, we can do the copy and replace
operations at the same time, which is quite faster for arrays when count=nothing.
- For dicts and sets, copying up-front is still faster as long as most original elements are preserved,
but for replace(), we can apply replacements directly instead of storing a them in a temporary vector.
- When the LHS of a pair contains a singleton type, we can subtract it from the element type
of the result, e.g. Union{T,Missing} becomes T.

Also simplify the dispatch logic by removing the internal _replace! method in favor of replace!.