input: Write a function to convert all characters of a given string to lowercase.
  
  assert to_lower("HELLO")=="hello"
  assert to_lower("TEST")=="test"
  assert to_lower("ALLCAPS")=="allcaps"
  assert to_lower("MixedCase")=="mixedcase"
  assert to_lower("")==""
output: def to_lower(s):
      return s.lower()