Improve Identifier API, and "undefined" errors
Improve Identifier API, and "undefined" errors

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.

epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless,epfl-lara/stainless