Rename key iterators to indicate they are implementation details of Key.

Rename 'key.KeyIterator' -> 'key._KeyIterator'

Move / rename 'iterator.KeyDataIterator' -> 'key._KeyDataIterator'.

Fixes #244.
Merge pull request #323 from tseaver/244-rename-key_data_iterator-not_api

Fix #244: Rename key iterators to indicate they are implementation details of Key.