Refresh and simplify AES256 encryption's implementation

[oleiade]

While looking into a security alert raised by CodeQL: https://github.com/oleiade/trousseau/security/code-scanning/1?query=ref%3Arefs%2Fheads%2Fmaster. I noticed the AES encryption's implementation felt a bit old-style, and could be greatly simplified and refreshed.

The scope of this issue is to kill to birds with one stone by:

• Addressing the potential overflow.
• Replacing the implementation to use the Gallois/Counter Mode, bcrypt for the passphrase, and the now standard Seal/nonce based methods.