HKDF about ironclad

[glv2]

The info vector seems to have the same role as the additional-data vector for argon2, therefore the additional-data keyword of the make-kdf function can be used.

I'd say:

• Put a info slot in the hmac-kdf class.
• In make-kdf, initialize the info slot with the value of additional-data when instanciating the hmac-kdf class.
• In shared-initialize, check that the info slot will be set to a value that makes sense.
• In derive-key, fetch the info slot and do stuff with it.

Basically, it should be pretty similar to what is done with additional-data for the argon2 class.

[glv2]

Implemented and merged (PR #20).