The big list to complete HPKE

[laurencelundblade]

While we have basic HPKE working in conformance with draft-ietf-cose-hpke-04, there's a lot before it is to commercial quality release level. Rather than filling a large number of issues, they are collected here.

• Split up the main HPKE encrypt entry point so that encode_headers can be used
• Split up the HPKE entry points so the final AEAD for one layer can be the same code as for AEAD for COSE_Recipient-based HPKE
• The big refactor of the main cose encrypt and decrypt to support one-layer COSE (right now it is only supported as a recipient)
• Re work to use struct q_useful_buf for all buffers and buffer operations
• Get rid of malloc
• Re work HPKE so it is works through the crypto adapter layer
• Add OSSL support through the crypto adapter layer
• Test (lots and lots of it)
• Documentation (lots and lots of it)