Merge branch '169-blake2' into 'master'

Switch KDF from SHA-256 to Blake2. #169

The BTP spec calls for Blake2s, but there's no Java implementation available. I suggest we go with Blake2b for now. If it turns out to be a performance bottleneck on 32-bit platforms we can consider implementing Blake2s and merging it upstream.

This depends on !13.

See merge request !21