Make type parms of "? super X" choose generators that produce potentially supertypes of X

[pholser]

Right now, for example, if you had a theory parm of type List<? super Number>, it would almost certainly produce items for the list that aren't conformant.

With a ? super X type parameter, we need to be able to find a generator that produces objects known to be either X or a supertype of X, and ensure that we don't allow mixed types.