Add slime-output-streams contrib

From lisp, you should be able to do something like:

    (defparameter *s* (swank::make-buffer-output-stream :buffer-identifier))
    (format *s* "Hello, World!~%")
    (force-output *s*)

And see "Hello, World!" in a buffer named
`*slime-target :buffer-identifier*` with the point at the beginning of
the next line.