Make SignVector's use a streaming cipher under the hood.

**Problem**
The new `signvectors` are a poor mans implementation of streaming ciphers, maybe hiding security problems that don't need to be there.

**Improvement**
It would be better if the `Writers`, `Readers` and `SignVectors` would be based on a streaming cipher such as [chacha](https://sodium-friends.github.io/docs/docs/streamencryption#crypto_secretstream_xchacha20poly1305_rekey).