Context manager `timer()`

# Add a decorator that will make timer() a context manager
@contextlib.____
def timer():
  """Time the execution of a context block.

  Yields:
    None
  """
  start = time.time()
  # Send control back to the context block
  ____
  end = time.time()
  print('Elapsed: {:.2f}s'.format(end - start))

with timer():
  print('This should take approximately 0.25 seconds')
  time.sleep(0.25)