Add an arity to a function

[benedekfazekas]

By default I define functions as

(defn func [arg]
  body)

Adding an arity to such a function requires

1. Wrapping the existing arglist and body in parens,
2. Inserting a set of parens, and
3. Defining a new arglist.

This is the most tedious refactoring I regularly do, and seems like a good candidate for a clj-refactor operation. I don't have any suggestions for how such an operation ought to work; any automated system ought to beat doing it manually.

Originally crated by @exupero on clj-refactor.el: clojure-emacs/clj-refactor.el#349

[benedekfazekas]

@bbatsov perhaps creating enhancement and refactor labels may make sense for this and future issues?

[bbatsov]

Done.