input: Write a function to replace all occurrences of a given character in a string with the provided replacement character and return the modified string. If the string is empty, return it as it is.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("No changes needed", 'a', 'b')=="No changes needed"
  assert replace_all_occurrences("Python programming", 'o', 'X')=="PYthXn prXgramming"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("Replaced all characters", 'R', 'S')=="Seplaced all characters"
output: def replace_all_occurrences(s, char, replacement):
      return s.replace(char, replacement)