Closes #30: Add sly-selector-map bound to C-c C-s by default

* sly.el (sly-selector-map): New keymap.
(sly-prefix-map): Bind it to "C-c C-s" by default.
(sly-inferior-lisp-buffer): New interactive function/helper.
(sly-start): Use it.
(sly-events-buffer): Made interactive, too.
(sly-db-pop-to-debugger-maybe): New function.

* contrib/sly-mrepl.el (sly-mrepl): Add "~" and "r" to `sly-selector-map'
(sly-mrepl--insert-prompt): Extract code to new `sly-db-pop-to-sldb-buffer-maybe'

* contrib/sly-scratch.el (sly-scratch): Add "s" to `sly-selector-map'

* contrib/sly-trace-dialog.el (sly-trace-dialog): Add "T" to `sly-selector-map'