Migrate from yojson to jsont and hkdf to kdf.hkdf
- ocaml-oci/src/spec: Convert all JSON handling from yojson to jsont
- mirage-crypto/tests/wycheproof: Replace ppx_deriving_yojson with jsont codecs
- ocaml-matter, ocaml-spake2, ocaml-hap, ocaml-csrf: Update dependency from
standalone hkdf package to kdf.hkdf (digestif-based implementation)
- ocaml-hap: Add hkdf_sha512 helper using kdf.hkdf API
- Various code cleanups for jsont compatibility