Move sly-buttons.el to lib/sly-buttons.el
Begin converting SLDB to more uniform emacs idioms

* sly.el (sldb-mode): SLDB buffer must be read only.
(sldb-var-number-at-point): Removed.
(sldb-local-part): New button type and constructor.
(sldb-inspect-var): Removed.
(sldb-insert-locals): Insert a `sly-local-part' button.

* contrib/sly-mrepl.el (sly-mrepl): Use `button-type-put' on
`sldb-local-part' buttons as well.
(sldb-copy-down-to-repl): No longer interactive.