Enumerating permutations in-place

Hi there!

For one of my projects I wrote a small implementation of [Heap's algorithm](https://en.wikipedia.org/wiki/Heap%27s_algorithm), which iterates through all permutations of an array by shuffling its elements in-place. It does so in a way that minimizes the number of moves.

I think this would deserve a place in Combinatorics.jl, but I can't figure out how to provide a useful generic implementation. Enumerating permutations in-place is only good if you do something at each step, so I'm not sure a standard iterator is the right choice. Right now, I'm leaning towards accepting a function as argument, which would be applied after each move. What do you think?