Add a method to produce a SecretKeyFactory from another factory deterministically.

[fjarri]

Should we do it just by label? Or label + salt, to add randomness?

[cygnusv]

My understanding of the SKF abstraction is that the seed input is assumed of high entropy. Therefore, salt shouldn't be necessary (or at most, optional).

[fjarri]

If it's not necessary, I'd rather not add an option if I can help it.

As for the implementation, I suppose I'll go the same way as secret_key_by_label() currently works, except deriving however many bytes SKF needs - is that secure enough?