Unimplementable ("Sealed") Traits

Hi,
a lot of times you want to have a trait in your library/crate that can't be implemented by the library users.
either because it's a marker trait for FFI related things, related to a custom type system, or because it's very highly unsafe and weird.

libstd's solution to this problem is exposing pub traits but marking them unstable to implement i.e. https://doc.rust-lang.org/std/slice/trait.SliceIndex.html

I propose we add something like #[non_exhaustive] but for traits, i.e. #[unimplementable]
we can already see similiar weird tricks in the wild, i.e.
https://docs.rs/zerocopy/0.2.8/src/zerocopy/lib.rs.html#207
https://github.com/rust-bitcoin/rust-secp256k1/blob/master/src/context.rs#L10
and more.

Start with the issue discussion and the linked SliceIndex, zerocopy, and rust-secp256k1 examples. Compare how each prevents downstream implementations, then specify the proposed attribute's semantics and unresolved trade-offs through the RFC process; done means the design is sufficiently detailed for review.