This is a new dependency resolver that produces valid install plans.
It works in polynomial time however because the search space is 
exponential in size it is not guaranteed to find a solution even if
one exists. It works by generating and then exploring the search
space represented as a lazy tree. It uses constraints to prune
choices and heuristics when guesses are necessary. Currently it can
generate install plans for 99% of the packages on hackage. The
remaining 6 packages should be doable with two extra tricks.
It is not finished and is not yet usable in practice.