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.

  which Leon repaired so I understood what the problem was.

unit, integration, and regression tests can now share code as they are
compiled together.

Unit tests are now in package leon.unit and shared code lives in
leon.test.