Fix show for Ufixed

This fixes a bug that cropped up in the display of methods written in terms of `T<:Ufixed`. Since you can't call `nbitsfrac` on a `Ufixed` (it needs to be a `UfixedBase`), it triggered an error.

I see there was a definition like this already, but I'm generally very against redefining how types or other low-level things are printed.