v0.6.0

• Fixed swap_columns() fast-path breaking data()
• Added spans
  • Added span
  • Added is_span<>
  • Added table::subspan() and table::const_subspan()
  • Added span_type<> and const_span_type<>
• Added soa_type<>
• Added const_iterator_type<>
• Added const_row_type<>
• Added column_indices<>
• Added soagen::for_each_column()
• Added row::for_each_column()
• Added generic names first, second, ..., sixteenth for unnamed columns 0-15
• Binary size improvements
• Documentation improvements