Integrating it the "wrong" way into common/crypto_ed25519.c passes
`make check`, and there appear to be some known answer tests for this,
so I assume I got it right.

Blinding a public key goes from 139.10 usec to 70.78 usec using
ed25519-donna (NB: Turboboost/phase of moon), though the code isn't
critical path, so supporting it is mostly done for completeness.