[slime-devel] REPL-specific printer control variables

This patch adds two variables to swank: *listener-print-level* and *listener-print-length*, analogous to the standard Lisp variables *print-level* and *print-length*, used to control how much of a form's return values get printed at the REPL but not of things printed directly by user code. If they're bound (by default, they aren't) then swank uses them when preparing the presentation of an evaluation's result for sending to Emacs. When they're unbound, the values of the standard CL depth and length variables instead.

I've written this because of one too many "oops, I forgot to wrap that form in a (progn ... nil)" mistakes on my part. Not a fatal error, but it can be inconvenient.