input: Write a function that converts a given string to its lowercase version. The function should iterate through each character in the string, convert each character to lowercase, and then return the resulting lowercase string.
  assert str_to_lower("Hello World")=="hello world"
  assert str_to_lower("PYTHON IS FUN")=="python is fun"
  assert str_to_lower("1234!@#")=="1234!@#"
  assert str_to_lower("")==""
  assert str_to_lower("ALREADY LOWERCASE")=="already lowercase"
output: def str_to_lower(s):
      return s.lower()