Makes the name match the standard library equivalents.
C++17 introduces non-member empty() which allows for nicer handling
in generic contexts. May as well make the data structure compatible with
it.
The only valid objects to add to the list are those that inherit from
IntrusiveListNode. Therefore anything being added to the list that isn't
inheriting from it will cause compilation to fail.
This generalizes the regular iterator to be compatible with both use
cases. Passing in the list instance directly isn't needed, because the
only way you'd ever get a valid instantiation of an iterator is from a
list instance itself.
