structure for unordered_multimap/unordered_multiset that uses an extra
pointer per node but makes most operations more efficient when equivalent
nodes are present.
Now uses macros instead of template metaprogramming to implement the
differences between containers with equivalent and unique keys.
Removed the erase_iterator stuff which complicated matters more than it helped.
Now just using pointers with some helper functions.
[SVN r2950]
The standard doesn't add them for local iterators, but I'm guessing that's an
oversight. So for now, I've hidden them behind a preprocessor condition. Which
I'll probably remove before review.
[SVN r2944]
unordered_multiset. I thought the proposed resolution for issue 528 allowed
them to be removed, but re-reading it, it only allows signatures that are
'otherwise indistinguishable', and they are not.
[SVN r2923]
