Despite that the library now appears at the top level in the filesystem and
namespace, it is still not intended to be used by Boost users - the library
is purposed as an internal tool for Boost needs.
The old headers are still present and define the old namespace as an alias
to the new namespace. Thus backward compatibility is preserved. These headers
will be removed as soon as all dependent libraries are updated.
While at it, also made some nonessential cleanup in headers.
