Make C-c C-z and C-c C-~ available in all of SLY

* contrib/sly-mrepl.el (sly-mrepl): C-c C-z and C-c C-~ are useful
in `sly-mode-map' too.
(sly-mrepl--with-repl-for): Add edebug spec.
(sly-mrepl-sync-package-and-default-directory): Work even when no
directory and or package can be inferred. Add interactive
spec.
(sly-mrepl--eval-for-repl): Add docstring.

* contrib/slynk-mrepl.lisp (eval-for-mrepl): Fix typo.
(sync-package-and-default-directory): PACKAGE-NAME and DIRECTORY
are now keyword args.