It was originally introduced because of some issues with Boost.Test and older
compilers, neither of which I'm using now. Simplifies a few things.
[SVN r79352]
Helps allocators which can't use incomplete pointers, and avoid using
base pointers where that might not be possible. And some other
reorganization. Storing arguments to emplace in a structure when
variadic template parameters aren't available. Changed some of the odd
design for working with older compilers.
[SVN r74742]
