I must've written the comment one way, and then changed the context
string later, without realizing that I'd copied it. Apologies to
everyone whose time I wasted with this.
Fixes https://github.com/BLAKE3-team/BLAKE3/issues/15.
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.
