Rename destructure-case -> slime-dcase.

That keeps the namespace clean, even though it's unlikely that
anybody else would have used that name.
Do the analog destructure-case -> dcase renaming on the CL side
to keep the names similar.

* slime.el (slime-dcase): Renamed from destructure-case. Update
users accordingly.
* swank.lisp (dcase): Renamed from destructure-case. Update users
accordingly.