Better idea for freezing sly-mrepl prompts, still incomplete

* sly.el (sly-channel-send): Friendlier error message on unsupported operations.

* contrib/sly-mrepl.el (sly-mrepl): Don't use sldb hooks.
(sly-mrepl--frozen-prompt-overlay)
(sly-mrepl--sldb-find-mrepl-buffer, sly-mrepl--freeze-prompt)
(sly-mrepl--unfreeze-prompt): Deleted.

* contrib/sly-mrepl.el (sly-mrepl--frozen-prompt-overlays)
(:freeze-prompt)
(:unfreeze-prompt): Manage prompt freezing with a hash table.

* contrib/swank-mrepl.lisp (mrepl-eval): Send :freeze and :unfreeze messages.
Fix sly-mrepl's history and package navigation

* contrib/sly-mrepl.el (sly-mrepl--insert-returned-values): Don't
increment the result counter if no results are reported.

* contrib/swank-mrepl.lisp (mrepl-eval): Call MREPL-EVAL-1.
(mread-eval-1): Renamed from REPL-EVAL. Fixed handling of
*PACKAGE*.
toquash

* swank-mrepl.lisp (mrepl-eval): Call MREPL-EVAL-1.
(mread-eval-1): Renamed from REPL-EVAL. Fixed handling of
*PACKAGE*.