Combine combinations and with_replacement_combinations

It might be worthwhile to combine the `combinations` and `with_replacement_combinations` functions. I could see a potential API as
```julia
combinations(x, n) # No replacement
combinations(x, n, replace=true) # With replacement
```
Having separate functions was probably worthwhile in The Olden Days, when keyword arguments were a performance killer and value-based dispatch with `Val` didn't exist.