Make maps and sets more usuable

Map, Set, WeakMap and WeakSet are all implemented by doing linear searches of an array of key values. The key values are compared with a routine that requires concrete values. Comparing abstract values is quite a bit more challenging, since more than one match may be present, each with different conditions. Some of these conditions can be removed by refining the abstract keys with the current paths, but others will have to be dealt with by returning a result which is a conditional abstract value.