Implement assoc_vector
Use assoc_vector in inverted_index_storage
Use vector instead of unordered_map
Make get_max_id method to key_manager and use it in inverted_index_storage

size method of key_manager is not equal to max id in it because a user may remove some ids in key_manager.
When a user remove an id, it causes segmentation fault.
Use fixed_size_heap instead of sort
Fix msgpack format for assoc_vector
Use const_cast in find method
Only compare keys in find method and operator []
Add random test for assoc_vector
Merge pull request #45 from unnonouno/fast-inverted-index

Fast inverted index