No more Emacs 23 gotchas.

* README.md (Emacs 23 gotchas): Remove section.
Three assorted slime-repl.el fixes (see details)

* contrib/slime-repl.el (slime-repl-emit): Bind `inhibit-read-only' to t
when inserting output. Might clash with read-only parts left over
from previous REPL session. Reported by Paul Bowyer.

(slime-repl-history-pattern): Allow M-p in any part of the REPL
buffer. Reported by Attila Lendvai.

(slime-sync-package-and-default-directory): Use `buffer-live-p',
don't just check that `buffer' is non-nil. Fails when the slime
process buffer has already been killed but connection lives on.
Unbreak "copy-to-repl" functionality in slime-presentations.el

The `slime-copy-presentation-to-repl' function simply copied a
presentations text to the REPL, which made these objects
ephemeral. For *inspector* the presentation was mispointing as soon as
the user navigated away*. For *sldb* the presentation it just didn't
work.

For this functionality, slime-presentation.el uses the same strategy
as slime-repl.el. A new `sldb-copy-down-to-repl' is also offered in
slime-repl.el.

* contrib/slime-presentations.el (slime-copy-presentation-to-repl):
Unbreak. Use same `slime-repl-send-string' strategy as
slime-repl.el.

* contrib/slime-repl.el (sldb-mode-map): Bind M-RET to
`sldb-copy-down-to-repl' in `sldb-mode-map'.
(sldb-copy-down-to-repl): New function, much like
`slime-inspector-copy-down-to-repl'