input: Write a python function to replace all occurrences of a substring with another substring in a given string.
  assert replace_substring("hello world", "world", "there") == "hello there"
  assert replace_substring("hello hello", "hello", "hi") == "hi hi"
  assert replace_substring("abcdabcd", "a", "z") == "zbcdzbcd"
output: def replace_substring(s, old, new):
      return s.replace(old, new)