The previous version of this API called for a key of exactly 256 bits.
That's good for optimal performance, but it would mean losing the
use-with-other-algorithms property for applications whose input keys are
a different size. There's no way for an abstraction over the previous
version to provide reliable domain separation for the "extract" step.
Putting secret keys on the command line is bad practice, because command
line args are usually globally visible within the OS. Even if these
flags are mostly intended for testing and experimentation, we might as
well do the right thing. Plus this saves people the trouble of hex
encoding their keys.