Still a little broken when called from an error that just happened in
the REPL itself.

* contrib/sly-mrepl.el (sly-mrepl--eval-for-repl): Accept new argument
`out-of-band' to eval in channel but in caller's thread.
(sldb-copy-down-to-repl): Pass `out-of-band' to
`sly-mrepl--eval-for-repl'.

* contrib/swank-mrepl.lisp (:compile-toplevel): Import FIND-CHANNEL.
(eval-in-mrepl): New function.