Fix sly-mrepl's history and package navigation

* contrib/sly-mrepl.el (sly-mrepl--insert-returned-values): Don't
increment the result counter if no results are reported.

* contrib/swank-mrepl.lisp (mrepl-eval): Call MREPL-EVAL-1.
(mread-eval-1): Renamed from REPL-EVAL. Fixed handling of
*PACKAGE*.