input: Write a function to convert a given string to its lowercase version. The function should iterate through each character in the string, convert it to lowercase, and concatenate it to a new string which is then returned.
  
  assert str_to_lower("Hello World")=="hello world"
  assert str_to_lower("PYTHON PROGRAMMING")=="python programming"
  assert str_to_lower("a1b2C3")=="a1b2c3"
  assert str_to_lower("")==""
  assert str_to_lower("Already Lowercase")=="already lowercase"
output: def str_to_lower(s):
      result = ""
      for char in s:
          result += char.lower()
      return result