input: Write a function to convert all characters of a given string to lowercase.
  assert to_lower("HELLO")=="hello"
  assert to_lower("aBCdeF")=="abcdef"
  assert to_lower("123ABcD")=="123abcd"
  assert to_lower("")==""
  assert to_lower("HELLO WORLD")=="hello world"
output: def to_lower(s):
      return s.lower()