Fix some fallout from the previous two commits

* sly.el (sly-macroexpansion-minor-mode-map): Use defvar.

* contrib/sly-mrepl.el (sly-mrepl-shortcut-key): Move down to "comma shortcut" section.
(sly-mrepl-reset-shortcut-key): Check for `sly-mrepl-shortcut-key' with boundp.