Font-lock function names in letfn

It'd be nice if function names in `letfn` were font-locked as function names. Here's a simple code snippet:

``` clojure
(letfn [(twice [x]
           (* x 2))
        (six-times [y]
           (* (twice y) 3))]
  (println "Twice 15 =" (twice 15))
  (println "Six times 15 =" (six-times 15)))
```
