Add a method to deserialize SecretKeyFactory from an externally created bytestring

[fjarri]

This is useful for the cases where one wants to derive SecretKeyFactory deterministically. One could use from_bytes() (because any 64 bytes will lead to a valid SKF), but it is better to make it a separate explicit method, to emphasize that the bytes did not come from an earlier SKF serialization. SecretKeyFactory.from_secure_random_bytes()? We will also need to expose the required bytestring length somehow.

[cygnusv]

In the context of umbral, 32 byte long input seeds are sufficient, assuming they have high entropy. What I'm not sure is how to support longer input seeds.

[fjarri]

For now we can limit the seeds to 32 bytes, possibly allowing larger seeds in the future - this won't break backward compatibility. For larger inputs, could we just use an HKDF?

One thing that I am not sure about: if we require high-entropy bytes (and pass them directly to the constructor), should we call it "seed"? To me, that implies that it can be something like a password, too. Should we emphasize the high-entropy requirement in the method name? Like from_secure_random_bytes() above, or something along these lines?