RFC: add SubVector (fast 1d slices)

Here's a baby step towards making Arrays just pointers + size & stride information. With it I'm getting ~2x - 2.5x speedups over `mapslices` in a recent application that looks a lot like this:

        notd = setdiff([1:nd], d)
        indexes[d] = 1:size(A, d)
        for c in Counter(szA[notd])
            indexes[notd] = c
            s = SubVector(Aout, indexes...)
            filt!(b, a, s)
        end

`Counter` comes from `Grid.jl`.