New method for subsets of fixed size?

[ettersi]

I want to iterate over all unordered pairs in a collection A. Thanks to this package, the easiest way to do this is to use subsets(A,2), but this seems rather wasteful as it does not exploit that I know at compile time the size of the set. So I suggest to add a method subsets(xs,::Type{Val{k}}) which can do so.

I'd be happy to provide a PR, but I first wanted to check that there's a chance it will make it into this package. Also, it would be my first PR ever, so I would likely need some help along the way.

[iamed2]

The new suggested way to do type constants would be to define subsets(xs, ::Val{k}) but yes this sounds like a good addition we would welcome.

[ettersi]

Would it be acceptable to introduce a dependency on StaticArrays? Having a mutable statically sized data type would come in handy.

[iamed2]

This was added in #13