[slime-devel] miscellaneous small additions

This patch adds a few minor tweaks I found useful: * slime-repl-{clear-buffer,clear-output}: clear the last and entire output in the *slime-repl* buffer * slime-documentation: pop up a buffer with a symbol's documentation instead of its description, if found. * tweak to completion, taken from ilisp, to complete filenames inside strings. * slime-set-default-directory: also set *slime-repl*'s default-directory, so e.g. find-file makes sense. /s