This is an owned sendable linked list which allows insertion and
deletion at both ends, with fast traversal through iteration, and fast
append/prepend.
It is indended to replace the previous managed DList with exposed list
nodes. It does not match it feature by feature, but DList could grow
more methods if needed.
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`.
