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.