Change Clause to an abstract class, and merge ConcreteClause into it. CompositeClause now
overrides defaults provided in Clause which are more suitable for composition of clauses.
Rework clauses so that rather than defining match/no-match behaviour themselves, they are
now composed by nesting them within clauses that understand how to match their child clauses.
This unifies a lot of the structure of clauses and removes corner cases needed for the first
design, as well as moving towards a model which is easier to prove.
Change away from extending ClauseVerifier for contracts which support clauses, and explicitely call
clause verification code in the verify() function. This should make the flow of control easier to understand.
