reimplemented BuiltinList and separated ListUpdate to a different class
re-re-implemented BuiltinList.Builder; fixed frame variable computation of AC collection; fixed BuiltinList JUnit test
fixed concurrent modification exception on pendingElements; added TODO comment
implemented a naive unification algorithm for BuiltinLists
renamed Collection#size() to Collection#concreteSize()
proved inorder, preorder, and postorder examples
moved conversion between IntSeq and List to list_pattern.k
proved tree_to_list_recursive
Merge pull request #872 from yilongli/reimplement-Java-backend-BuiltinList

Reimplement java backend builtin list