Add high-level HPKE API

[franziskuskiefer]

The HPKE API is the (hac)spec API right now, which isn't great to use.
Add a high-level, usable API on top.

[keks]

I am working with that API right now, and I agree it is not great to use. I think my main trouble is that it uses type aliases over newtypes, and that these type aliases are not public. This means that the functions just return giant tuples of values that have no semantics attached to them.

Should there be a usable API on top, or should we change the Rust code and re-validate it in hax?

[keks]

Another problem: The ContextS_Seal function takes a context and then returns it again. That makes it really awkward to call from a &mut self-style method of a struct that has the context as a field, because we can't move the context out of there. My current workaround is creating a new context on the stack and then using mem::swap before and after the call to ConetxtS_Seal.

[keks]

It would also be nice if the error implemented core::error::Error

[franziskuskiefer]

The HPKE implementation here really is a spec. That's fine. But it hasn't really been used.
We should drop the HPKE implementation in here (it still lives in the https://github.com/cryspen/hpke-spec repo, that version just doesn't build anymore because the hacl dependencies changed) and instead use https://github.com/franziskuskiefer/hpke-rs. We just need to figure out the best way to integrate the two.

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[franziskuskiefer]

We should get #523 in.

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