Use Fix from data-fix package

[phadej]

A proposal to use data-fix's Fix in unification-fd.

data-fix is somewhat widely used, and the next recursion-schemes will move to use it as well. It would be great if unification-fd also moved to use the same type.

There are small changes to instances. I'd like to change UTerm's Show instance to use Show1 type (and maybe Eq and Ord instances, like they are written for Free in https://hackage.haskell.org/package/free-5.1.3/docs/Control-Monad-Free.html#t:Free)