Fix problems when reading from mREPL

* contrib/sly-mrepl.el (sly-mrepl--accept-process-output): New function.
(sly-mrepl--insert-prompt, :set-read-mode): Use it.
(sly-mrepl--read-mode): Renamed from
`sly-mrepl--expect-sexp-mode'
(:set-read-mode): Unset then reset read-only when reading from
user. Call `sly-mrepl--catch-up'.
(sly-mrepl--insert-output, sly-mrepl-return): use
`sly-mrepl--read-mode'
Fix another read bug

* sly-mrepl.el (sly-mrepl-return): Go to end before
submitting input to read, and don't save it in history.