It was not obvious that the KT128/KT256 customization string can be
used to set a key, or what it was designed to be used for at all.

Also properly use key_length and not digest_length for the BLAKE3
key length (no practical changes as they are both 32, but that was
confusing).

Remove unneeded simd_degree copies by the way, and that doesn't need
to be in the public interface.