Support unsafely implementing trait, bypassing derive

Rust for Linux would like to manually implement certain traits, providing a manual justification of soundness. We can't support a manual `unsafe impl` because this locks us into a forwards-compatibility guarantee that we won't add non-defaulted items to the trait. Instead, we'll need to provide an unsafe-to-call macro, a `#[derive(unsafe(zerocopy::...))]`, or some other form of indirection.

Start by reviewing zerocopy's trait and derive support, focusing on how manual implementations and unsafe derives are currently handled. Compare the proposed unsafe-to-call macro, unsafe derive form, and other indirection options; done means a documented, maintainable mechanism that permits manual soundness justification without promising never to add non-defaulted trait items.

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