Use normal `sly-rex' RPC when inspecting objects from MREPL

Also organize code in contrib/swank-mrepl.lisp

* contrib/sly-mrepl.el (:inspect-object): Removed, replaced by
usage of a slyfun.
(sly-mrepl-part): Use `sly-eval-for-inspector'

* contrib/swank-mrepl.lisp (eval-for-mrepl): New SLYFUN. Replaces
`eval-in-mrepl'.
(inspect-entry): New SLYFUN. Replaces the :inspect-object channel
method.
(:inspect-object): Deleted channel method.