[Proposal] Add interactive function switch-to-mrepl

Currently C-c C-z switches to the lisp inferior process, which is not of much use. Because there might be multiple REPL's I think a simple solution is to just return the last used one.

IMHO the binding in lisp-mode-map should also be overridden.

```elisp
(define-key lisp-mode-map (kbd "C-c C-z") 'sly-mrepl-switch-to-mrepl)
```