Allow for-each loop for `ObjectNode`s

(note: PR exists -- https://github.com/FasterXML/jackson-databind/pull/1590 -- but probably needs some changes)

So: 2.x API exposes `Iterator` approach, but that is bit clumsy. API should either be changed (which would be sub-optimal since this does not have to break), or augmented (probably preferable) to allow use of `Iterable` instead, both for field names and entries.