input: Write a python function to convert a string to its hexadecimal representation.
  assert stringToHex('hello') == '68656c6c6f'
  assert stringToHex('world') == '776f726c64'
  assert stringToHex('1234') == '31323334'
output: def stringToHex(s):
      return ''.join(format(ord(c), '02x') for c in s)