Added a transform-diff function.

For convenience, added a transform-diff function:

(defun transform-diff (transform-To transform-From)
+ "Return the transform that, when left-multiplied, will convert transform-From into transform-To."
+ (transform* transform-To (transform-inv transform-From)))

This seems like a very common task when working with transforms, so it makes sense to have it in the core package, unless something like it is here already.

Shouldn't it be the way round?