Convert integer x into a string of bytes, as per X9.62.

def int_to_string( x ):
  """Convert integer x into a string of bytes, as per X9.62."""
  assert x >= 0
  if x == 0: return b('\0')
  result = []
  while x:
    ordinal = x & 0xFF
    result.append(int2byte(ordinal))
    x >>= 8

  result.reverse()
  return b('').join(result)