Fix copying calls to REPL

* contrib/sly-mrepl.el (sly-mrepl--eval-for-repl): Use `after-prompt'.
(sly-mrepl--eval-for-repl): Accept `before-prompt' and
`after-prompt' callbacks.
(sly-mrepl--copy-objects-to-repl): Pass `callback' as an
`after-prompt' callback.
(sly-mrepl--insert-call): Fix off-by-one.