Only build/use ed25519-donna.
More a long term thing so I don't forget about it, but once we are sure we like
ed25519-donna, we can/should remove the
ref10 code from builds, along with the paranoid initialization/runtime sanity checking, as well as only using the donna Curve25519 keypair generation code.
I'd be ok with keeping the code around as part of the test suite to catch mysterious breakage. Tentatively setting this to
0.2.8.x-final since I don't think enough people run the alpha build for us to catch discrepancies between the two codebases if any exist.