Omit parentheses in threading macros when optional

Parentheses are not required when using the threading macros for functions having no argument specified, so use them only when necessary.

```clojure
;; good
(-> x fizz :foo first frob)

;; bad; parens add clutter and are not needed
(-> x (fizz) (:foo) (first) (frob))

;; good, parens are necessary with an arg
(-> x
    (fizz a b) 
    :foo
    first 
    (frob x y))
```