Handle \ and /

[oxinabox]

This is more of #3.
But these operation I always find hard to reason about.

It might be able to reuse the machinery for working out multiplciation names,
I am not sure

[iamed2]

If it helps for figuring out what names should be, you can think of A \ B = X being A * X = B, so the first dim of X is the second dim of A and the second dim of X is the second dim of B.