Improve/fix/use consistently features related to deep type hierarchies

src/main/scala/leon/codegen/CompilationUnit.scala

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

src/main/scala/leon/purescala/Definitions.scala

src/main/scala/leon/purescala/MethodLifting.scala

src/main/scala/leon/solvers/smtlib/SMTLIBSolver.scala

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

src/main/scala/leon/synthesis/utils/ExpressionGrammar.scala

src/main/scala/leon/termination/ChainComparator.scala

src/main/scala/leon/termination/StructuralSize.scala

src/main/scala/leon/utils/TypingPhase.scala