Force Aead::AeadImpl to be Send + Sync

[nox]

When trying to use AeadCtxR/AeadCtxS in a generic context which requires Send or Sync, downstream users need to add a bound to Aead::AeadImpl even though this is a doc(hidden) item and thus an implementation detail that may change afterwards.

All implementations of Aead provide an AeadImpl type that is already Send + Sync so it doesn't hurt to add a trait bound there.

[rozbb]

Thank you for bringing this up! Do you have a minimal example by any chance that works after this PR but not before? I was trying to see how far you could get with AeadCtx in threads after this PR, and I can't make it work.

I added the following code to examples/client_server.rs

fn do_export<A, K1, K2>(ctx: AeadCtxR<A, K1, K2>)
where
    A: hpke::aead::Aead,
    K1: hpke::kdf::Kdf,
    K2: hpke::kem::Kem + Send + Sync,
{
    // Wrap the context to prevent lifetime issues
    let wrapped = std::rc::Rc::new(ctx);
    // Do an export() (which doesn't require mut) in another thread
    std::thread::spawn(move || {
        let mut buf = [0u8; 32];
        wrapped.export(b"foobarcontext", &mut buf);
    });
}

And I get the following error

error[E0277]: `Rc<AeadCtxR<A, K1, K2>>` cannot be sent between threads safely
   --> examples/client_server.rs:110:24
    |
110 |       std::thread::spawn(move || {
    |       ------------------ ^------
    |       |                  |
    |  _____|__________________within this `{closure@examples/client_server.rs:110:24: 110:31}`
    | |     |
    | |     required by a bound introduced by this call
111 | |         let mut buf = [0u8; 32];
112 | |         wrapped.export(b"foobarcontext", &mut buf);
113 | |     });
    | |_____^ `Rc<AeadCtxR<A, K1, K2>>` cannot be sent between threads safely
    |

[nox]

Rc<T> is never Send, you want Arc<T>:

fn do_export<A, K1, K2>(ctx: hpke::aead::AeadCtxR<A, K1, K2>)
where
    A: hpke::aead::Aead + 'static,
    K1: hpke::kdf::Kdf + 'static,
    K2: hpke::kem::Kem + Send + Sync + 'static,
{
    // Wrap the context to prevent lifetime issues
    let wrapped = std::sync::Arc::new(ctx);
    // Do an export() (which doesn't require mut) in another thread
    std::thread::spawn(move || {
        let mut buf = [0u8; 32];
        wrapped.export(b"foobarcontext", &mut buf);
    });
}

[rozbb]

Apologies for the delay. Thank you!