Introduce TypeDepths, filter grammars, bound size of NdProgram

src/main/scala/leon/purescala/TypeTreeOps.scala

src/main/scala/leon/synthesis/rules/Cegis.scala

src/main/scala/leon/synthesis/rules/CegisLike.scala

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

src/main/scala/leon/utils/SeqUtils.scala