Add support to only perform an s2k key derivation once while encrypting/decrypting multiple private keys

[lubux]

In previous versions, it is not possible to encrypt multiple private keys with the same encryption key derived from a passphrase. A fresh encryption key is derived for each encryption and similar s2k key derivations are repeated for decryption. This commit adds support for encrypting/decrypting multiple private keys with a single encryption key derived with a single s2k key-derivation:

• Adds two high-level methods on Entity called EncryptAllKeys and DecryptAllKeys

• Adds two functions in packet called PrivateKeysEncrypt and PrivateKeysDecrypt that allow encrypting/decrypting multiple PrivateKeys with the same passphrase.

• Refactoring in PrivateKey encryption and decryption

• Adds Cache in the s2k package to avoid recomputing similar key derivations in packet.PrivateKeysDecrypt

• Changes the type of the s2k.Params salt field to byte array and introduces a getter for retrieving the salt.

[lubux]

Thanks, I applied the suggestions.