Synchronize prop224 spec with implementation
In our ed25519 key blinding code we have a few pieces that are not in the spec. At the very least we have the following constant strings that get hashed, which are not mentioned in the spec:
const char str = "Derive temporary signing key"; ... const char str = "Derive temporary signing key hash input";
We should eye the implementation for any other unspecified parts, and bake them in the spec.