Prettyprint TwoContexts constraints.
Fix TwoContexts to make use of the JMEq

Right now things don't work as they should, since translating

    JMEq ctx type1 t1 type2 t2

to

    Conj [Unify ctx set type1 ctx set type2, Unify ctx type1 t1 ctx type2 t2]

is somehow worse than

    Unify ctx set type1 ctx set type2 :>>: Unify ctx type1 t1 ctx type2 t2

which means that the dependency tracking for constraints is off.