Closes #140: More stable interface for copying things to REPL

* contrib/slime-presentations.el (slime-copy-presentation-to-repl): Use
`slime-repl-copy-part'.

* contrib/slime-repl.el (slime-repl-copy-part)
(slime-inspector-copy-down-to-repl): Use `slime-repl-copy-part'.

* contrib/slime-repl.el (slime-repl-copy-part): New function.

* contrib/swank-repl.lisp (listener-eval-for-part): New slimefun.