Move some utils to new lib/sly-messages.el file

* sly.el (sly-messages): Require new lib.
(sly-message, sly--last-message, sly-temp-message, sly-warning)
(sly-error, sly-display-oneliner, sly-oneliner)
(sly-completing-read-function, sly-completing-read)
(sly-flash-inhibit, sly-flash-region): Move to new
lib/sly-messages.el lib

* lib/sly-buttons.el (sly-messages): require new lib.

* lib/sly-messages.el: New file.