Implement `KnownLayout` for all `T: Sized`

This is currently impossible because it causes impl conflicts with any generic `#[derive(KnownLayout)]`. However, if we could get Rust to understand that we're only emitting a derive for unsized types, it'd be possible. Prototyped in https://github.com/google/zerocopy/commit/5601ac4821528dfa72548d324d2a2a329459cfb8, but Rust doesn't know that `TrailingField: KnownLayout` implies `TrailingField: !Sized`. Maybe we can spell `TrailingField: !Sized` somehow?