Handle multi-line history entries in sly-mrepl

* contrib/sly-mrepl.el (sly-mrepl-new): No need to do this, it's a one off
error if ~/.sly-mrepl-history does not exit.
(sly-mrepl-new): Use new `sly-mrepl-read-input-ring'
(sly-mrepl-history-separator): New variable
(sly-mrepl-read-input-ring): New function
Consistently load ~/.swankrc (user init file) from both loaders

* swank-loader.lisp (load-user-init-file):
Moved to swank.lisp.
(load-site-init-file): Deleted, seemed useless.
(load-swank): Don't call SWANK::BEFORE-INIT.
(setup): Don't load init files here.

* swank.asd (:asdf): Use IN-PACKAGE.
(perform :after load-op): call SWANK::INIT

* swank.lisp (start-server): Don't call INIT here.
(before-init): Delete funcion.
(init): Shared between the two loaders, warns about unimplemented
interfaces once, always loads user init file, runs *after-init-hook*
Remove some useless references to defunct presentations

* swank.asd (:swank/sbcl-pprint-patch)
(:swank-presentation-streams, :swank-contribs): Remove references
to presentations. Some still remain in the manual...