Need unit tests badly.

* sly-mrepl.el (sly-mrepl--insert-output): Don't check for
`sly-mrepl--busy-p'. Take optional `face' parameter. Use
`sly-mrepl--break-output-p'
(sly-mrepl--break-output-p): New function.
(sly-mrepl--dedicated-stream-output-filter, :write-string): Insert
output in `sly-mrepl-output-face'