Make `IndexMove` private and unsafely implement it for all lists

[marmeladema]

In some sense it acts as a get_unchecked method but is not exposed publicly anymore to prevent rogue implementations.

[dwrensha]

If I understand correctly, the purpose of this is to make it so that iterator-based access to capnproto lists avoids bounds checks. Have you experimentally observed a significant improvement from this change? I'm somewhat reluctant to add more instances of unsafe like this.