input: Write a function to replace all occurrences of a character in a string with the provided replacement character. If the given string is empty then return the empty string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'X')=="HellX WXrld!"
  assert replace_all_occurrences("Python", 'P', 'X')=="Xython"
  assert replace_all_occurrences("", 'a', 'X')==""
  assert replace_all_occurrences("Hello", 'z', 'X')=="Hello"
output: def replace_all_occurrences(s, char, replacement):
      return s.replace(char, replacement)