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.