input: Write a python function to reverse a given string using slicing.
  assert reverse_string('Hello World!') == '!dlroW olleH'
  assert reverse_string('Python') == 'nohtyP'
  assert reverse_string('') == ''
  assert reverse_string('racecar') == 'racecar'
output: def reverse_string(s):
      return s[::-1]