Identifier fields which can be private are now private.
Identifier now inherits Ordered.
Define Undefined error, and its subclass SolverUndefinedError. Use it
mainly in solvers.

Solvers are no longer distinguished in 20 traits depending on what they
implement. It turns out that most leon solvers already implemented
everything:

1) Being interrupted
2) Push / Pop
3) checkAssertions/getUnsatCore (a naive implementation of these
   can be added by mixing NaiveAssumptionSolver in)

- 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.

- enumSimple allows to enumerate distinct models (models will typically
  only slightly differ , e.g. only w.r.t one integer)

- enumVarying allows to specify a caracteristic (i.e. `size`) and
  enumerate `N` models of each size.

- solvers are not freed directly, they are reclaimed by their
  solver factory

- reset() is used when available to reset a solver instead of creating a
  fresh one.

- Custom pattern matching through unapply functions.
- TypeParameter now has private constructor, can only be created
  with TypeParameter.fresh or _.freshen
- Stricter type checking for expressions. Correct various type errors.
- Remove some redundant code.

Methods can be overriden in subclasses
Non-root classes can define methods
Deep hierarchies are supported

In case of PortfolioSolver, the Exception should be handled correctly
through Future magic