Improve on current in-VM AEAD implementation

Now that we have a basic initial implementation of the in-VM AEAD scheme, it would be good to improve on it with at least the following two points:

- [ ] #2434
- [ ] Add support for associated data (AD).

The second point is mostly localized to changing our implementation in the standard/core library, and this should be more or less straightforward. The first point is more involved as we need to prove (or at least be convinced) that the Sponge2-inspired padding rule doesn't degrade any properties of our AEAD scheme.