[boost] [container] Forward declarations for scoped allocator traits

Hi Ion, I was just looking at the scoped allocator support. A container currently needs to include all of scoped_allocator.hpp just to customize the appropriate traits which seems a bit much. Could you declare (not define) constructible_with_allocator_suffix and constructible_with_allocator_prefix in a separate header so that they can be specialised without including all the other things? I've also attached a patch that fixes a couple of typos that I noticed. Daniel