Unordered: Overhaul the implementation.
Store nodes in a single linked list, with hash values so that their
buckets can be found when needed. Iterators now only have to store a
pointer to the node and don't have to iterate over empty buckets to
reach the next node. This allows the container to meet the iterator
requirements - fixing the speed issues with equal_range and erase.
Also, define iterators in their own namespace, so that they don't
accidentally pull in detail functions via ADL.
I've simplified the code slightly by removing some of the special
cases for empty containers. Renamed a few things as well and other
minor changes that were made as I went along.
