Adding AD rules for backends via ext

[kylebeggs]

Is this something the package would consider merging? As an exploratory task, I put together some rules for Mooncake and got a 10x speedup in the pullback. I used this as my test bed

function loss(points)
    tree = KDTree(points)
    _, dists = allknn(tree, 3)
    return -sum(sum, dists)
end

[KristofferC]

A useful extension seems like it would indeed be a good idea.