Fix customization of `sly-mrepl-shortcut-key'

* sly-mrepl.el (sly-mrepl-shortcut-key): Add a `:set'
specification, otherwise the variable's useless.
(sly-mrepl-reset-shortcut-key): New function.
(sly-mrepl--shortcut-location-p): Only attempt to search for "`"
if the shortcut key is ","
Closes #18: rework sly-macroexpansion-minor-mode keybindings

Also make the macroexpansion buffer read-only.

* sly.el (remap): Remove obsolete toplevel remappings block.
(sly-macroexpansion-minor-mode-map): Setup remaps here.