message_headers is now a set of partial class template
specializations instead of a template class alias. This solves
a problem where template functions taking message_headers as a
parameter could not deduce argument types, since std::conditional
obscured the deduced context.
Both classes are refactored to share declarations using an #ifdef,
to eliminate an ugly set of extra declarations needed when building
the documentation.
Copy and move class special members are added.
A new function message::base() is provided which returns the
message_headers portion of a message.
