input: Write a function to convert a given string to its lowercase version. The function should iterate through each character of the string, convert each character to lowercase, and then concatenate the results to form the final lowercase string. If the input string is empty, the function should return an empty string.
  assert str_to_lower("Hello World!")=="hello world!"
  assert str_to_lower("PYTHON")=="python"
  assert str_to_lower("numb3rs")=="numb3rs"
  assert str_to_lower("TIC_TAC_TOE")=="tic_tac_toe"
  assert str_to_lower("")==""
output: def str_to_lower(s):
      result = ""
      for char in s:
          result += char.lower()
      return result