input: Write a function to convert a given string to its lowercase version. The function should iterate through each character of the string, convert it to lowercase, and then return the new lowercase string.
  assert str_to_lower("Hello World!")=="hello world!"
  assert str_to_lower("Python PROGRAMMING")=="python programming"
  assert str_to_lower("ALL CAPS")=="all caps"
  assert str_to_lower("mixed CASE 123")=="mixed case 123"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      return s.lower()