def num_align(n, a)
      case a
      when 1, 2, 4, 8
        bits = a - 1
        n + bits & ~bits
      else
        raise "Unsupported alignment"
      end
    end
def num_align(n, a)
      case a
      when 1, 2, 4, 8
        bits = a - 1
        n + bits & ~bits
      else
        raise "Unsupported alignment"
      end
    end
Create a new marshaller, setting the current packet to the
 empty packet.
 Round _n_ up to the specified power of two, _a_