input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. If the character is not found in the string, return the original string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'o', 'a')=="Pythan Programming"
  assert replace_all_occurrences("Java", 'z', 'b')=="Java"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("C++", 'C', 'A')=="A++"
output: def replace_all_occurrences(s, char, replacement):
      return s.replace(char, replacement)