def flags
      mask = @size - size
      flag = []
      flag << :non_main_arena unless (mask & 4).zero?
      flag << :mmapped unless (mask & 2).zero?
      flag << :prev_inuse unless (mask & 1).zero?
      flag
    end
def flags
      mask = @size - size
      flag = []
      flag << :non_main_arena unless (mask & 4).zero?
      flag << :mmapped unless (mask & 2).zero?
      flag << :prev_inuse unless (mask & 1).zero?
      flag
    end
The chunk flags record in low three bits of size
 @return [Array<Symbol>] flags of chunk
 @example
   c = [0, 0x25].pack("Q*").to_chunk
   c.flags
   # [:non_main_arena, :prev_inuse]