Use a separate package for swank-gray.lisp.

Use gray streams also for CMUCL, SCL, and ABCL.

* swank-backend.lisp (gray-package-name): New interface function.
(import-from): Deleted. No longer used.
(*gray-stream-symbols*): Moved to swank-gray.lisp.
* swank-abcl.lisp, swank-cmucl.lisp, swank-scl.lisp
(gray-package-name): Implemented and delete old implementations of
make-input-stream and make-output-stream.
* swank-allegro.lisp, swank-ccl.lisp, swank-clisp, swank-ecl.lisp,
swan-lispworks.lisp, swank-sbcl.lisp,  (gray-package-name):
Implemented.
* swank-gray.lisp: Make and use swank-gray package for this file.
* swank-loader.lisp (*sysdep-files*): Load swank-gray for CMUCL,
SCL, and ABCL.