Functions for evaluating the form (not an atom) that the point is
currently in, and for evaluating the symbol underneath the point. Both
require smartparens to work.
Also, add all evil-leader bindings to lisp-interaction-mode and declare
prefixes.
Helm seems to treat "!" specially in pattern matching, so having a ! in
the pattern string when traversing directories is problematic. This
change fixes#2737, because as far as I can tell "+" has no special
meaning in a helm pattern.
Of course, we can choose a different character, but I'm fond of "+" as
representing "more layers here".
