input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. If the string is empty, return the string as it is.
  
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Goodbye!", 'o', 'a')=="Gaadbye!"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("Mississippi", 's', 'Z')=="MiZZiZZipi"
  assert replace_all_occurrences("ABABABA", 'A', 'B')=="BBBBBBB"
output: def replace_all_occurrences(s, char, replacement):
      return s.replace(char, replacement)