Both extra::treemap::TreeMap and extra::treemap::TreeSet have
corresponding iterators TreeMapIterator and TreeSetIterator.
Unfortunately, the tests and extra::serialize use the older .each.
Update all the dependent code, and remove .each.
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
The removed test for issue #2611 is well covered by the `std::iterator`
module itself.
This adds the `count` method to `IteratorUtil` to replace `EqIter`.
