add `swank:clear-repl-variables`

null implementation (we don't have any repl variables)
add `swank:apropos-list-for-emacs`

and also `swank:describe-definition-for-emacs`, helping to make it
useful.  It's still a bit weird with the confusion between name of
thing, name of help file, and so on (and also because
describe-definition-for-emacs must return a definition kind from a
restricted set defined in base slime) but it's basically working.