input: Write a python function to replace all occurrences of a substring in a string with a new substring.
  assert replace_all(orig_str="Today is a beautiful day", pattern="a", replacement="the") == "Todthey is the beutiful dthe"
  assert replace_all(orig_str="hello world", pattern="o", replacement="a") == "hella warld"
  assert replace_all(orig_str="mississippi", pattern="iss", replacement="ox") == "moxoxippi"
output: def replace_all(orig_str, pattern, replacement):
      return orig_str.replace(pattern, replacement)