`const fn` support

In `crypto-bigint` we've had to build an entirely parallel set of types in order to support `const fn`: [`ConstChoice`](https://docs.rs/crypto-bigint/0.6.0-rc.0/crypto_bigint/struct.ConstChoice.html) and [`ConstCtOption`](https://docs.rs/crypto-bigint/0.6.0-rc.0/crypto_bigint/struct.ConstCtOption.html), which are `const fn`-friendly versions of `subtle`'s equivalent `Choice` and `CtOption`. It would be nice to have upstream support for this in `subtle` some way.