These changes adjust the `hy-mode` key-map and make use of `use-package`'s
`:mode` and `:interpreter` settings.
Regarding key-maps, the REPL prefix `s` has been added alongside `e`, since `s`
seems at least as conventional among layers as `e`. Although `e` is used for
Elisp and Clojure, it isn't used for Python or Racket. Scheme appears to use
both, so there's some precedence for the approach used here.
