input: Write a function to replace all occurrences of a given string with an alternate string.
  assert replace_string("Hello World!", "World", "Python") == "Hello Python!"
  assert replace_string("Welcome to the World of Programming", "World", "Universe") == "Welcome to the Universe of Programming"
  assert replace_string("Hello", "World", "Python") == "Hello"
  assert replace_string("", "World", "Python") == ""
  assert replace_string("Python is fun, Python is cool", "Python", "Java") == "Java is fun, Java is cool"
output: def replace_string(s, old, new):
      return s.replace(old, new)