More sly-mrepl fixes to make it behave like classic SLIME repl.

* contrib/sly-mrepl.el (sly-mrepl--insert-output): Don't always
insert a newline.
(sly-mrepl--ensure-newline): New function.
(define-channel-method :prompt): Merge with `sly-mrepl--prompt'.
(sly-mrepl--prompt): Deleted.
(sly-mrepl--insert-returned-values): Use `sly-mrepl--ensure-newline'
(define-channel-method :write-values): Call `accept-process-output' just before inserting
return values.
(sly-mrepl--debug-overlays, sly-mrepl--debug)
(sly-mrepl--turn-on-debug, sly-mrepl--turn-off-debug): New debug
section.

* contrib/swank-mrepl.lisp (mrepl-eval): Fix bug. UNWIND-PROTECT
wasn't doing anything.