Remove non-used optional gray stream methods.

* swank-gray.lisp (stream-read-char-will-hang-p)
(stream-line-length): Deleted.
Make the ugly deadlock workaround even more ugly.

* swank-sbcl.lisp (stream-force-output): Create swank-gray package
early enough.
* swank-backend.lisp (gray-package-name): Fix docstring.
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.