
  -spec byte_size(Bitstring) -> non_neg_integer()
                     when Bitstring :: bitstring().

  Returns an integer that is the number of bytes needed to contain 
  Bitstring. That is, if the number of bits in Bitstring is not
  divisible by 8, the resulting number of bytes is rounded up.
  Examples:

    > byte_size(<<433:16,3:3>>).
    3
    > byte_size(<<1,2,3>>).
    3

  Allowed in guard tests.
