Peeking `usize::MAX` with `peek_nth`

[Takashiidobe]

Running this playground, it looks like you can't peek_nth with an argument of usize::MAX without panicking.

In a way, this makes sense cause peek_nth gets you an item with index in 0..usize::MAX, so getting the index usize::MAX itself is out of bounds. However, if you peek_nth past the end, this implementation returns None. I assume the implicit contract is that it should always return an option, and not panic.

It's pretty straighforward to special case usize::MAX and just return None if its provided as an argument to peek_nth. Otherwise, it can be kept as panicking and noted in the docs as an example.