input: Write a function to reverse an input string. The function should take a string as input and return the reversed string.
  assert rev_string("Hello World!")=="!dlroW olleH"
  assert rev_string("Python")=="nohtyP"
  assert rev_string("12345")=="54321"
  assert rev_string("")==""
  assert rev_string("a")=="a"
output: def rev_string(s):
      return s[::-1]