Streaming signed data

[kirelagin]

This is split from #3.

Unlike in #3 we want to attach a signature to each chunk of the stream, so it can be verified and consumed in a single pass.

This is not implemented in Sodium for signing, but is implemented for encryption, so we can do something similar (and also be sure to check with https://saltpack.org/signing-format-v2 to see if there are any caveats).