Add a test to check stack overflow in "fist-drop" section

[tamuhey]

I think it is great to add a test to this section to cause stack overflow before implementing our own Drop.

[kolulu23]

You can try this in the playground. refering to this post

[zenyusy]

tried a bit: playground-2 versions of drop

found that without impl Drop the prog would go stack overflow; with either version of drop it can run successfully.

can i say that both versions of drop are correct ?

while let Some(mut b) = self.head.take() { self.head = b.next.take(); }

while let Some(b) = self.head.take() { self.head = b.next; }