Enhance the add arity refactoring

[anthonygalea]

Just came across a case which the current clojure-add-arity refactoring does not handle well i.e. when a defn lies within a reader conditional. This pull request modifies clojure-add-arity to support this case.

• The commits are consistent with our [contribution guidelines][1].
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
• You've run M-x checkdoc and fixed any warnings in the code you've written.
• You've updated the changelog (if adding/changing user-visible functionality).
• You've updated the readme (if adding/changing user-visible functionality).

[bbatsov]

We'll have to mention the bugfix in the changelog as well. Btw, does the command handle properly letfn and things like proxy, reify, defprotocol?

[anthonygalea]

Can you think of any other cases @bbatsov?

[yuhan0]

What about defmacro? And to some extentdefmethod, although adding an arity to a single method probably isn't very useful.
Also the anonymous fnform can take multiple arities and would be nice if it was supported too :)

[anthonygalea]

Many thanks for your feedback @yuhan0. Wasn't even aware that it was possible to have multi-arity in a defmethod.

[bbatsov]

Nicely done! Now I've started wonder if we need a remove arity counterpart. :-)

[anthonygalea]

Hadn't thought of that. Guessing it will add very little value, but we'll never know until it's implemented. I'll give it a shot.