Release v1.6.1

Add X-Wing to the readme and bump version.

Release v1.5.0

Preparing for release v1.4.0

Preparing CIRCL release v1.3.9

Serializing ciphertext with 32-bit prefixes.

Notice about ciphertext change and testing format.

Previously, tkn20 ciphertext was encoding the ciphertext header
`C1`, the envelope `env` (containing inner ciphertext), and
macData using 16-bit prefixes, which caused a limitation on
the maximum size allowed for encrypting plaintexts.

With this change, the encoding now uses 32-bit prefixes for
these three elements allowing to encrypt plaintexts longer
than 2^16 bytes. So, ciphertexts produced by tkn20 package are
now 12 bytes longer.

Ciphertexts in the previous format are still decryptable.
The following functions are backwards-compatible:
 - AttributeKey.Decrypt
 - Attributes.CouldDecrypt
 - Policy.ExtractFromCiphertext

Releasing CIRCL v1.3.7

Releasing CIRCL v1.3.6