input: Write a function to replace all occurrences of a character in a string with the provided replacement character. If the input string is empty, return the empty string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Programming is fun!", 'm', 'Z')=="PrograZZing is fun!"
  assert replace_all_occurrences("Python", 'y', 'a')=="Phathon"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("Hello World!", '!', '?')=="Hello World?"
output: def replace_all_occurrences(s, old, new):
      return s.replace(old, new)