Streaming HKDF API?

The `Context` struct, in theory, allows an allocation-free implementation. But in practice the implementation allocates behind the scenes, because the HKDF API doesn't support streaming. It would be nice (albeit very low priority) to examine whether there's an easy way to fix it.

Start by reading the Context struct and the HKDF API usage in its implementation. Determine whether a streaming HKDF interface can remove the hidden allocations while preserving the existing CPace behavior; the work is done when the allocation-free path is demonstrated and covered by the relevant tests.

Written by the indexing model from the issue text.