Introduce MutableTree

src/main/scala/leon/codegen/CodeGeneration.scala

src/main/scala/leon/evaluators/RecursiveEvaluator.scala

src/main/scala/leon/evaluators/StreamEvaluator.scala

src/main/scala/leon/purescala/Expressions.scala

src/main/scala/leon/purescala/Types.scala

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

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