Unordered: Merge unordered from trunk.
- Activate `std::allocator_traits` for gcc 4.7 and Visual C++ 11.
- Implement variadic construct in `boost::unordered::detail::allocator_traits`
when variadics, rvalue references and SFINAE expression are available.
- Use variadic construct from `allocator_traits`, or when not available move
the logic for constructing `value_type` to a lower level, so the container
code is a bit simpler.
- Avoid `-Wshadow` warnings. Fixes #6190.
- Implement `reserve`. Fixes #6857.
[SVN r78432]
Daniel James