Consider creating a poseidon::Transcript primitive.

[therealyingtong]

(analogous to poseidon::Hash)

[therealyingtong]

This may introduce a circular dependency between halo2 and the (eventual) poseidon crate:

poseidon::Transcript will have to implement the halo2::transcript::{Transcript, TranscriptRead, TranscriptWrite} traits, i.e. will have a halo2 dependency; however, the halo2 crate has to import poseidon::Transcript to instantiate a transcript.

[daira]

halo2 wouldn't necessarily have to import poseidon::Transcript if it were a type parameter provided by the prover and verifier.