Fix sly-mrepl's freezing/unfreezing logic

* contrib/sly-mrepl.el (sly-mrepl-mode): Overlay should be pushed on
insertion before.
(sly-mrepl--insert-output): Another case for a "\n" isnertion.
(sly-mrepl--stacked-errors, :freeze-prompt, :unfreeze-prompt):
Delete.
(:prompt): Takes new `error-level' and `condition' args.
(sly-mrepl--prompt-face): Take `error-level' arg.

* contrib/swank-mrepl.lisp (declass mrepl): New PENDING-ERRORS slot.
(swank::drop-unprocessed-events): Don't SEND-PROMPT from here.
(mrepl-eval): Redesigned and simplified (hopefully).
(mrepl-eval-1): Renamed from MREAD-EVAL-1 (which was a typo)