Wait for REPL to be setup in REPL-popping operations

* sly.el (sly-dispatch-event): Must be %s, not %d.

* contrib/sly-mrepl.el (sly-mrepl--with-repl-for)
(sly-mrepl--call-with-repl): New macro and helper function.
(sly-mrepl--eval-for-repl)
(sly-mrepl-sync-package-and-default-directory): Use
`sly-mrepl--with-repl-for' macro.