Constraints should "produce" values.

Allow constraints to be queried for specific values. Eg. if you have an AttrChain

```
NonTerm{2}.attribute 
```

and you want to know what set of values a constraint allows it have ask the constraint

```
obj = # the model of NonTerm{2}.attribute
values = constraint.produce(objs, obj)
# values is now the set of values the constraint requires the value of the obj to
# be in.
```
