adding existentials and conjunctions to BLD consequents

In planning to implement RIF for N3, the two extensions I mostly need
are to allow existentials and conjunctions in the the rule consequents.
These are straightforward extensions, with nice fallback rewrites, but I
thought I would just raise the question of putting them directly in BLD.

I note, in particular, that Production Rule systems want something very
much like existentials in the consequent, too, so this might possibly
even go into Core.  But maybe it's not exactly the same thing, and
datalog certainly doesn't have it.

I'm fine working with this as just an extension, but I thought I should
at least raise the issue in case anyone else is sympathetic.

     - Sandro