input: Write a function to replace all occurrences of a specified character in a given string with a provided replacement character. Return the modified string.
  assert replace_all_occurrences("Hello World!", 'l', 'X')=="HeXXo WorXd!"
  assert replace_all_occurrences("Python Programming", 'o', 'a')=="Pythan Pragramming"
  assert replace_all_occurrences("Coding is fun", ' ', '-')=="Coding-is-fun"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("12345", '5', '0')=="12340"
output: def replace_all_occurrences(s, old, new):
      return s.replace(old, new)