Adhere to new contrib loading mechanism

* slime-hello-world.el (slime-hello-world): Add/remove to/from
slime-mode-hook rather than activating global mode.
(slime-hello-world-on): Delete.
(slime-global-hello-world-mode): Delete.
(slime-hello-world--buffer): New function.
(slime-hello-world): Use async eval and pop to buffer for hello
worlds.

* swank-hello-world.lisp: provide :SWANK-HELLO-WORLD

* slime-hello-world.asd: Delete (was wrong file name)

* swank-hello-world.asd: Added