It is sometimes useful to encrypt data under some symmetric key.
While this was possible to do using passphrase-derived keys, there was
no support for long-term storage of the keys that was used to encrypt
the key packets.

To solve this, a new type of key is introduced. This key will hold a
symmetric key, and will be used for both encryption and decryption of
data. Specifically, as with asymmetric keys, the actual data will be
encrypted using a session key, generated ad-hoc for these data.
Then, instead of using a public key to encrypt the session key, the
persistent symmetric key will be used instead, to produce a, so to say,
Key Encrypted Key Packet.

Conversly, instead of using a private key to decrypt the session key,
the same symmetric key will be used. Then, the decrypted session key
can be used to decrypt the data packet, as usual.

As with the case of AEAD keys, it is sometimes useful to "sign"
data with a persistent, symmetric key.

This key holds a symmetric key, which can be used for both signing and
verifying the integrity of data. While not strictly needed, the
signature process will first generate a digest of the data-to-be-signed,
and then the key will be used to sign the digest, using an HMAC
construction.

For technical reasons, related to this implenetation of the openpgp
protocol, the secret key material is also stored in the newly defined
public key types. Future contributors must take note of this, and not
export or serialize that key in a way that it will be publicly availabe.

Since symmetric keys do not have a public and private part, there is no
point serializing the internal "public key" structures. Thus, symmetric
keys are skipped when serialing the public part of a keyring.

Generate an AEAD subkey when requesting an HMAC primary key.