Summary: Enable channel messages even while in debugger

Need for multithreaded channels was much exaggerated.

* contrib/sly-mrepl.el (listener): Invert order of args in call to
`sly-mrepl--prompt'
(sly-mrepl--prompt): Invert order or args to match channel method.

* contrib/swank-mrepl.lisp (create-mrepl): Don't return package
info.

* lib/lisp/swank.lisp (sldb-loop): Process :EMACS-CHANNEL-SEND
messages here as well.
(spawn-channel-thread): Improve docstring.
(define-channel-method :teardown): Maybe handle singlethreaded
channels case (needs testing).