Reworked lambda equality for models returned by underlying solvers

src/main/scala/inox/ast/SymbolOps.scala

src/main/scala/inox/solvers/smtlib/SMTLIBTarget.scala

...main/scala/inox/solvers/unrolling/TemplateGenerator.scala

src/main/scala/inox/solvers/unrolling/UnrollingSolver.scala

src/main/scala/inox/solvers/z3/AbstractZ3Solver.scala