Implement optional iterator features

[kpreid]

• FusedIterator Implement and require FusedIterator #29
• ExactSizeIterator, at least for fieldless enums.
• Iterator::size_hint()
• DoubleEndedIterator — or not, because that has costs to implement.
• Iterator::fold() and Iterator::try_fold() overrides.

Each of these should have tests, and documentation for the conditions under which it exists.

[kpreid]

These should also be opt-in (controlled by derive helper attributes or separate derive macros) to ensure that dependents don't accidentally break semver by changing the types of private fields whose properties were exposed.