Before layers were sometimes only activating the non strict
version of smartparens. Also some were only disabling
the non-strict version leaving some of the smartparens
advices intact.
With this PR, all layers set the right version of smartparens.
Also if the layer is trying to disable it a standard function
will now take care to disable all versions of smartparens.
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.
