input: Write a function to replace all occurrences of a character in a string with the provided replacement character. If the string is empty, return the string as is.
  assert replace_all_occurrences("Hello World!", 'l', 'X') == "HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'Z') == "HellZ WZrld!"
  assert replace_all_occurrences("", 'a', 'b') == ""
  assert replace_all_occurrences("Python is fun", 'n', 'z') == "Pythoz is fuz"
  assert replace_all_occurrences("No change", 'x', 'y') == "No change"
output: def replace_all_occurrences(s, char, replacement):
      return s.replace(char, replacement)