ed25519 needs SignStream and VerifyStream functions

[noloader]

Current ed25519 signing and verification does not lend itself to signing large messages, like a 4.4 GB file (a typical LiveCD ISO). Pre-hashing the message could allow existential forgeries, so we probably want to avoid letting users hash the message M' = H(M) and then using M' as the message to be signed or verified.

That leaves us two choices. First, modify ed25519 signing and verification for RFC 8032's ed25519ph. Second, add a stream overload that reads through the stream in blocks and feeds it to the hash by repeatedly calling Update in place of Update(m, mlen).

I don't want to modify things for RFC 8032 just yet. It looks like the IETF changed the algorithm beyond using dom2 and PH(M), if I am parsing the RFC sections 5.1.6 and 5.1.7 correctly.

I think that leaves the overload that takes an istream in place of byte* message, size_t messageLength.