This method returns an iterator over mapped windows of the starting
iterator. Adding the more straight-forward `Iterator::windows` is not
easily possible right now as the items are stored in the iterator type,
meaning the `next` call would return references to `self`. This is not
allowed by the current `Iterator` trait design. This might change once
GATs have landed.

The idea has been brought up by @m-ou-se here:
https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Iterator.3A.3A.7Bpairwise.2C.20windows.7D/near/224587771

This means one iteration is amortized O(1) instead of O(N). Note that
`N` here refers to the array size (not the iterator length!) which is
expected to be fairly small most of the time.

Unfortunately, this optimization makes the implementation a lot more
involved and requires lots more `unsafe` code. Therefore, quite a few
tests were added to hopefully make sure everything is safe and sound.