stable

Main improvements:

1) non pointer template type

• you can make use of shared_ptr for better value resource management
• to migrate existing implementations: change art::art<int> m to art::art<int*> m

2) carry key during iteration

for (auto it = m.begin(), it_end = m.end(); it != it_end; ++it) {
  std::cout << "key: " << it.key() << std::endl;
}

Acknowledgments:
Evance Soumaoro
River Xu