input: Write a function to replace all occurrences of a given character in a string with the provided replacement character. The function should return the modified string.
  
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Hello World!", 'o', 'Z')=="HellZ WZrld!"
  assert replace_all_occurrences("Hello World!", ' ', 'Y')=="HelloYWorld!"
  assert replace_all_occurrences("Hello World!", '!', '')=="Hello World"
  assert replace_all_occurrences("Hello World!", 'H', 'h')=="hello World!"
output: def replace_all_occurrences(s, char, replacement):
      return s.replace(char, replacement)