Fix history inspection in sly-mrepl

* contrib/sly-mrepl.el (sly-mrepl-part): New "describe" and "inspect" actions.
(listener): Set result-counter to -1.

* contrib/slynk-mrepl.lisp (with-eval-for-repl): New macro.
(eval-for-mrepl): Use it.
(inspect-entry): Use it.
(describe-entry, pprint-entry): New functions.