Make `Choice` and `CtOption` be `Hash`

While rare, sometimes it is convenient to use `CtOption` in hashmaps/sets. This PR suggests adding `Hash` to `CtOption` and `Choice` to allow that use case, see e.g. [`crypto-bigint::Checked`](https://github.com/RustCrypto/crypto-bigint/blob/master/src/checked.rs#L15). It also aligns with `std::option::Option`, which is also `Hash`.