iterable/iterator serializers

[pjfanning]

Copy code from 2.12 branch

[cowtowncoder]

I can't give much feedback but I do like avoiding having to wrap Scala types in JDK equivalents, and that little bit of insulation is probably a good idea for stability -- even if adding little bit of duplication.
So +1 for changes.

Mostly we need to figure out what would be a good way to get changes in jackson-databind "default" handling propagated to other modules, Scala module as well as various data type modules.

[pjfanning]

@cowtowncoder just as an example, as well as avoiding the conversion from Scala to Java collections (which can be cheap enough because in most case wrappers are used, instead of creating brand new collections), I've tried to avoid calls to get the collection size as that can be expensive in some scala collections (eg singly linked lists).