Rewrite "Implementing Vec" chapter - Unique is permanently unstable

[tesuji]

This chapter use Unique to demonstrate how to implement a Vec. But it is
permanently unstable and can only be used by std. Also in
https://github.com/rust-lang/rust/blob/478464570e60523adc6d303577d1782229ca1f93/src/libcore/ptr/unique.rs#L28-L34
it recommends that we should use NonNull instead and consider PhantomData<T>.

[eisterman]

What is the pattern to use instead of Unique? With NonNull?

[Coder-256]

I think this is a duplicate of #83.