message: Put VectorTreeIteratorBase in `internal` namespace
diff: namespace verible {
  +namespace internal {
  +
  // Class implementing common VectorTree*Iterator members using CRTP polymorphic
  // chaining. Derived class must implement following methods:
  // - `static VectorTreeType* _NextNode(VectorTreeType* node)` - returns pointer
  // to a next node
  template <typename ImplType, typename VectorTreeType>
  -class _VectorTreeIteratorBase {
  +class VectorTreeIteratorBase {
  public:
  using iterator_category = std::forward_iterator_tag;
  using difference_type = std::ptrdiff_t;
  @@ -36,8 +38,8 @@ class _VectorTreeIteratorBase {
  using pointer = VectorTreeType*;
  using reference = VectorTreeType&;
  - _VectorTreeIteratorBase() : node_(nullptr) {}
  - _VectorTreeIteratorBase(pointer node) : node_(node) {}
  + VectorTreeIteratorBase() : node_(nullptr) {}
  + VectorTreeIteratorBase(pointer node) : node_(node) {}
  reference operator*() const {
  CHECK_NOTNULL(node_);
  @@ -71,12 +73,14 @@ class _VectorTreeIteratorBase {
  pointer node_;
  };
  +} // namespace internal
  +
  template <typename VectorTreeType>
  class VectorTreeLeavesIterator
  - : public _VectorTreeIteratorBase<VectorTreeLeavesIterator<VectorTreeType>,
  - VectorTreeType> {
  + : public internal::VectorTreeIteratorBase<
  + VectorTreeLeavesIterator<VectorTreeType>, VectorTreeType> {
  using this_type = VectorTreeLeavesIterator<VectorTreeType>;
  - using base_type = _VectorTreeIteratorBase<this_type, VectorTreeType>;
  + using base_type = internal::VectorTreeIteratorBase<this_type, VectorTreeType>;
  public:
  VectorTreeLeavesIterator() : base_type() {}
  @@ -103,10 +107,10 @@ VectorTreeLeavesTraversal(VectorTreeType& tree) {
  template <typename VectorTreeType>
  class VectorTreePreOrderIterator
  - : public _VectorTreeIteratorBase<VectorTreePreOrderIterator<VectorTreeType>,
  - VectorTreeType> {
  + : public internal::VectorTreeIteratorBase<
  + VectorTreePreOrderIterator<VectorTreeType>, VectorTreeType> {
  using this_type = VectorTreePreOrderIterator<VectorTreeType>;
  - using base_type = _VectorTreeIteratorBase<this_type, VectorTreeType>;
  + using base_type = internal::VectorTreeIteratorBase<this_type, VectorTreeType>;
  public:
  VectorTreePreOrderIterator() : base_type() {}
  @@ -140,10 +144,10 @@ VectorTreePreOrderTraversal(VectorTreeType& tree) {
  template <typename VectorTreeType>
  class VectorTreePostOrderIterator
  - : public _VectorTreeIteratorBase<
  + : public internal::VectorTreeIteratorBase<
  VectorTreePostOrderIterator<VectorTreeType>, VectorTreeType> {
  using this_type = VectorTreePostOrderIterator<VectorTreeType>;
  - using base_type = _VectorTreeIteratorBase<this_type, VectorTreeType>;
  + using base_type = internal::VectorTreeIteratorBase<this_type, VectorTreeType>;
  public:
  VectorTreePostOrderIterator() : base_type() {}
  