Add sylvesters

* contrib/sly-mrepl.el (sly-mrepl): Add `sly-mrepl-on-connection' to `sly-connected-hook'
(sly-mrepl-pop-sylvester): New variable.
(sly-mrepl-on-connection): Renamed from `sly-mrepl-pop-to-mrepl'. Use `sly-mrepl-pop-sylvester'
(sly-mrepl-new): Pop a sylvester sometimes.
(sly-mrepl--sylvesters): New variable.
(sly-mrepl-random-sylvester): New function.