Version 0.3.0 (contains BC breaks)

• (#6) Sapient is no longer a subclass of the Guzzle HTTP client. BC Break
  • Instead, it expects an instance of a new AdapterInterface passed to the constructor, which allows multiple HTTP implementations to be used.
  • Future scope: Make this argument optional, default to a generic adapter (which does not yet exist).
• (#2) Public key encryption is now based on XChaCha20 instead of XSalsa20. BC Break
  • Our key and nonce derivation now use a similar construction to libsodium's crypto_kx
    • Difference: BLAKE2b output size is 56 bytes instead of 32
    • The first 32 are the key, the last 24 are the nonce
    • This maps a 32 byte shared secret and two 32 byte public keys, passed through a secure hash function, to a 56-byte pseudorandom output; this should prove to be secure so long as X25519 is secure