* sly-mrepl.el (sly-mrepl--insert-prompt): Wait a millisecond on W32

This fixes an apparently buggy `accept-process-output' on Windows
which sometimes leads to mixed output and return values in the mREPL.
Fix more newline bugs in sly-mrepl

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'
* sly-package-fu.el (sly-goto-package-source-definition): Use `sly-error'
* sly.el (define-button-type 'sly-xref): Tweak comment