Enforce where line breaks are allowed

[weavejester]

Some forms have conventions on where line breaks are permitted. For example:

;; ok
(if x y z)

;; ok
(if x
  y
  z)

;; bad
(if x y
  z)

For more complex forms, like defn, this might be affected by the type of arguments:

;; ok
(defn foo
  "docstring"
  [x y z])

;; less ok
(defn foo "docstring"
  [x y z])

Ideally we want some syntax that defines the positions where newlines are valid, and to add newlines that are missing. This issue has been opened to discuss what form that syntax will take.

This issue replaces issue #60, #61, #105, #317 and #412.