Handle pre-generated CRS

[swasilyev]

No description provided.

[burdges]

There are two questions here:

How does zcash use groth16::ParameterSource?

How big are these SRSs? If large, then we can even memmap the file I guess.

How should we associate auth_depth with the SRS? If we should support multiple auth_depths ala #10 then we could specify a directory that contains {auth_depth}.vpk and/or {auth_depth}.ppk files.

We'd then implement our own

pub trait RingVRFParameterSource : groth16::ParameterSource {
    fn auth_depth(&self) -> u32;
}

pub struct RingVRFVerifingKey {
    g16: groth16::VerifyingKey,
    auth_depth: u32,
}
impl ::core::ops::Deref for RingVRFVerifingKey {
    type Target = groth16::VerifyingKey;
    fn deeref(&self) -> groth16::VerifyingKey { &self.g16 }
}

[burdges]

Should we say CRS or SRS? I donno..

We now associate the depth to the SRS using the RingSRS type, so if you've code that loads the SRS it can determine the depth too, maybe using the file name. It looks messy right now because zcash makes their library kinda painful by never providing standard traits. We could work around this, but we should just dump zcash's libs for ZEXE and solve all these problems there.