`nthperm!` is not very ergonomic; add 3-arg variant?

`nthperm!(a, k)` overwrites `a` with the `k`th permutation. This is difficult to reason about if what one wants is to iterate over `k` and get the permutations of `a` without minimal allocations.

For instance, doing something like this is not a good idea:
```jl
julia> v = [1, 2, 3]
julia> for k in 1:6
println(nthperm!(v, k))
end
[1, 2, 3]
[1, 3, 2]
[3, 1, 2] # <---
[1, 2, 3]
[3, 1, 2] # <--- woops, seen this already
[2, 1, 3]
```
since it returns the same permutations multiple times (because `v` is being permuted).

I think a three-argument version of `nthperm!` would be nice. This could just piggy-back off the current 2-argument implementation:
```jl
function nthperm!(dst::Vector{T}, a::AbstractVector{T}, k::Integer) where T
nthperm!(copyto!(dst, a), k)
end
```

I suppose one could argue it's a trivial function, but it took me a while to realize this would be the right way to integrate with the 2-argument method.