The API documentation is currently sparse, but the API is fairly small and
descriptive.
The `ohttp` crate has the following features:
- `client` enables the client-side processing of oblivious HTTP messages:
encrypting requests and decrypting responses. This is enabled by default.
- `server` enables the server-side processing of oblivious HTTP messages:
decrypting requests and encrypting responses. This is enabled by default.
- `rust-hpke` selects the [hpke](https://github.com/rozbb/rust-hpke) crate for
HPKE encryption. This is enabled by default and cannot be enabled at the same
time as `nss`.