Support simplifying "U' d" to "y'"

[ittayd]

[lgarron]

I think that this is useful, but there are different ways of simplifying an alg. There are also various transformations you might want to do, such as eliminating the y and adjusting all following moves to compensate.

I think that this should either go into a separate mode or a separate app. It would be useful to know about core use cases for this!