* sly.el: Add sly-macroexpand

Oi,
   Currently there is no keybinding to macroexpand an expression once. I figured that adding a negative argument to C-c M-m would be a good place to add one.

I realize that this function is the same of ```sly-macroexpand-1``` expect it toggles the other way around so maybe you want to add the functionality another way to prevent duplicated code?