rif-core links to rif-bld

I had an idea for dealing with all those rif-core links to rif-bld.  I
think I'll make an editors-draft snapshot of BLD now, and make the links
point into that.  It's better than pointing to the wiki (they'll still
work in the future), or to the Last Call draft (losing all the anchors).
If anyone has any objections or better ideas, please speak up very soon.

     -- Sandro