refactor columns.hpp
refactor columns.hpp
use DuckTyping in abstract_column
Modify msgpack::object arguments to pass by references
Fix cpplint violations
Remove columns.cpp
Remove comented-out codes
Refactor abstract_column
Fix index conditions
Fix assignment of table::bit_vector
Fix bug of abstruct_column
Enable bit_vector_column to take msgpack::object
Enable save/load for column_table
Implement pack_with_index
Add insert to bit_vector_column
Implement dump