Version 3.2

Changes

• Mac, message authentication with a shared key: HMAC-SHA256, returned as URL-safe Base64.
  Until now the only thing a SharedKey could do was encrypt. Every MAC is made under a required
  context, and the MAC key is derived from the shared key with HKDF-SHA256 under that context,
  so one key can serve Crypt and any number of MAC purposes without one being able to forge for
  another.
• KeyRing, a map from key ID to SharedKey for rotation, when old and new clients call the
  same server for a while. KeyRing::fromPairs() reads pairs straight from configuration and
  skips a slot that is not filled rather than holding an empty key: a MAC under an empty key can
  be forged by anyone, which is an easy mistake to make with an unused "previous key" variable.
• Request signing: Request\RequestSigner and Request\RequestVerifier authenticate a whole
  HTTP request — method, path, timestamp and body — without ever sending the secret, which suits a
  credential both sides hold (a secret built into an app, or shared between two services) where a
  SplitToken, being a bearer token, does not. They take and return plain strings, so there is no
  framework dependency and no fixed header names. The verifier reports why it refused through the
  RequestVerificationFailure enum, meant for the log, never for the response. The acceptance
  window is 300 seconds by default; there is no nonce, which the README states plainly.
• Test vectors for clients in other languages, computed outside PHP:
  tests/fixtures/request-signing-vectors.json. The signed string and the key derivation are
  specified in the README.

Nothing existing changes: this release only adds classes.