This PR rewrites the getConstantValue implementation, to avoid bad performance arising from unnecessary recursion through newtype constructors. Previously, we had predicates

ConstantValue ExprCfgNode::getConstantValue();
ConstantValue Expr::getConstantValue();

defined (mutually) recursively by virtual dispatch. However, this meant we had to populate the newtype underlying ConstantValue as part of the recursion, since QL does not permit unbound newtypes, which in turn adds artificial (non-linear) recursion through the constructors in each predicate.

In the new implementation, we instead define the following three predicates, for each type T of ConstantValue:

predicate isT(ExprCfgNode n, T v);
predicate isTExprNoCfg(Expr e, T v);
predicate isTExpr(Expr e, T v);

and then the result of those is used to define the newtype, and the user-facing getConstantValue predicates, but only after the recursive constant propagation computation. (The isTExprNoCfg is not related to the performance optimization.)

One down-side of this approach is that we have to replicate logic for each type T, for example, isInt and isString both have to replicate how constants propagate through assignments. We may be able to get rid of some of this duplication using parameterized modules, though.