Allow for hybrid custom constraints

[haz]

If A and B are classes with a @proposition annotation, then we may want to be able to write a constraint like...

e = Encoding()

@proposition
class A:
    pass

@proposition
class B:
    pass

b = B()
e.add_constraint(~A | b)

Interpretation would be that every instance of A is substituted in for a new version of this constraint at compile time.

Probably needs some advanced handling of the operator overloading (so that the class is considered an nnf.Var object, and a regular nnf constraint is built no matter what the arguments are).

Note that this is a far more advanced version of #87