Improve batch verification API

[hdevalence]

Incorporates feedback from @FiloSottile.

• [x] fix the order of arguments in BatchVerifier.Add to match Verify;
• [x] made Add infallible, deferring failures to the batch verification;
• [x] rename BatchVerifier.VerifyBatch to BatchVerifier.Verify;
• [x] make Verify fail on an empty batch;
• [x] make Verify idempotent.

Closes #9

[FiloSottile]

Ping. If this is not ready to merge, I would revert the change that introduces the dangerous API.