Opaque return types inside `mina-signer` crate.

[berzanorg]

I would like to have a mina_signer::Schnorr object but mina_signer::create_kimchi returns an opaque type which is impl mina_signer::Signer<H>.

So I would like to construct a mina_signer::Schnorr object using hasher and domain_param properties but they are private properties and I can't use them in my crate.

Having opaque types kills usability in many contexts.

It would be great if either mina_signer::create_kimchi doesn't return an opaque type or hasher and domain_param properties of mina_signer::Schnorr struct becomes public.

Related file: https://github.com/o1-labs/proof-systems/blob/9f6d04e3b35f1c75419319a522b23aae4f991e12/signer/src/schnorr.rs#L96