First attempt at freezing sly-mrepl prompts from debugger

* sly.el (sldb-exit-hook): New hook
(sldb-exit): Run it.

* contrib/sly-mrepl.el (sly-mrepl--frozen-prompt-overlay)
(sly-mrepl--last-prompt-overlay): New variables.
(sly-mrepl--sldb-find-mrepl-buffer, sly-mrepl--freeze-prompt)
(sly-mrepl--unfreeze-prompt):  New functions.
(sly-mrepl--send-input): Highlight last prompt.
(channel-method :prompt): Unhighlight last prompt.
(sly-mrepl--last-prompt-beg-and-end, sly-mrepl--freeze)
(sly-mrepl--unfreeze): Delete.
(sly-mrepl--teardown-repls): Renamed from
`sly-mrepl-teardown-repls'
(sly-mrepl): Setup new sldb hooks.
Fix bug when killing a sly-mrepl buffer with a live sldb.

* contrib/swank-mrepl.lisp (define-channe-method :teardown mrepl):
New method.
(mrepl-eval): Don't send :evaluation-aborted message if mrepl is
being torn down.