Add unsafe/unchecked slicing

[emilk]

When the user has already done their own bound checking, they don't want to pay for it again. Therefore I propose the slice methods has an unsafe _unchecked version.

We used these methods in arrow2 extensively, and found the performance different to be quite significant for our use case.

[emilk]

After some internal review it's become clear that this ugly code-generated arrow-derserialization we do is never on the fast path (anymore), so we don't need this after all. Sorry for the noise!