New "describe" and "pretty print" (d and p) in frame locals

* sly.el (sly-db-local-variable): Two new button actions:
"describe" and "pretty print" for frame locals.

* lib/lisp/slynk.lisp (pprint-frame-var, describe-frame-var): New slyfun.
Fix problems when contribs try to ask if sly is connected

* contrib/sly-tramp.el (sly-tramp-to-lisp-filename): Ensure
`sly-connected-p' is not called if there is no current connection.