Introduce model minimization/maximization

- Measure is provided by an user-defined expression of BigInt type
  (e.g. List.size)

- Search for min/max lazily enumerates intermediate models. Users of the
  enumerator may wish to skip them using `.last`, and/or bound the
  search using `.take(N)`.

- Discovery of upper&lower bounds is done with pseudo-exponential
  progression from initial model. Bisection method is then used to
  zero-in on the min/max.