message: improve const correctness for patricia tree maps
  Summary: This patch marks the dereference operator on patricia tree map iterators as `const`.
diff: @@ -1052,12 +1052,14 @@ class PatriciaTreeIterator final {
  return !(*this == other);
  }
  - const std::pair<Key, mapped_type>& operator*() {
  - return *reinterpret_cast<std::pair<Key, mapped_type>*>(&m_leaf->m_pair);
  + const std::pair<Key, mapped_type>& operator*() const {
  + return *reinterpret_cast<const std::pair<Key, mapped_type>*>(
  + &m_leaf->m_pair);
  }
  - const std::pair<Key, mapped_type>* operator->() {
  - return reinterpret_cast<std::pair<Key, mapped_type>*>(&m_leaf->m_pair);
  + const std::pair<Key, mapped_type>* operator->() const {
  + return reinterpret_cast<const std::pair<Key, mapped_type>*>(
  + &m_leaf->m_pair);
  }
  private:
  