Unfinished tweaks to sly-mrepl freezing logic

* contrib/sly-mrepl.el (sly-mrepl--pending-requests): Removed.
(sly-mrepl-mode): Don't enter `font-lock-mode'.
(sly-mrepl-new): Prompt-sending is done exclusively remotely.
(sly-mrepl-prompt-face): New face.
(sly-mrepl--prompt): Unfreeze previous prompt. Propertize inserted
prompt.
(define-channel-method :copy-to-repl): regorganize.
(sly-mrepl--debug): Remove, relatively useless...
(sly-mrepl--dummy-sender): Remove.
(sly-mrepl--send-pending): Remove.
(sly-mrepl--send): Simplify.
(sly-mrepl--last-prompt-beg-and-end): New local variable and
accessor.
(sly-mrepl--freeze, sly-mrepl--unfreeze): New functions,
reasonably useless for now.
(sly-mrepl--send-input): Freeze prompt before sending.

* contrib/swank-mrepl.lisp (create-mrepl): Use WITH-LISTENER and
SEND-PROMPT.
Guess package at point in sly-mrepl

* contrib/sly-mrepl.el (sly-mrepl-mode): Set
`sly-find-buffer-package-function'.
(listener): Don't set `sly-buffer-package', pass `package' to
`sly-mrepl--prompt'.
(sly-mrepl--prompt): Take extra arg package and set
`sly-mrepl--prompt' property value to it.
(sly-mrepl-guess-package): New function.