utilize separate prepareConstraints pass in UnionSolve solver
utilize separate prepareConstraints pass in UnionSolve solver

Ignore-this: 8f8e6df2938cd0f7e5afb6ddf035bcfa

darcs-hash:6b5fba5ff204e305ef282d4e93c2eefb1f32fa2a

dpwright/jhc,dpwright/jhc,dpwright/jhc,dpwright/jhc