input: Write a function to reverse a given string. The function should take a string as input and return the string with its characters in reverse order.
  assert reverse_string("hello")=="olleh"
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("")==""
  assert reverse_string("12345")=="54321"
  assert reverse_string("racecar")=="racecar"
output: def reverse_string(s):
      return s[::-1]