Tracking issue: Implement FROST

[hdevalence]

FROST is a threshold signature and DKG construction by @chelseakomlo and Ian Goldberg.

Link: https://crysp.uwaterloo.ca/software/frost/

In terms of the API, I think it would make sense to have the FROST implementation in a frost submodule separated from the main API, and feature-gate the entire submodule on a frost feature.

• Rebase WIP: Frost implementation #26 onto latest
• Use zkcrypto/jubjub's SubgroupPoint so as to be closer to 1:1 with the reference impl's Ristretto prime order group assumptions
• Triple-check the 'general' Schnorr sig divergence of (z, c) vs (R, c)

[hdevalence]

In order to remain unopinionated, the messages generated while running the FROST protocols should be Serde'able, so that they can be serialized using whatever method is convenient for the library user. This will require either adding Serde support to jubjub and bls12_381 (allowing the use of derive for the messages) or implementing custom derives for the message types in redjubjub.

[hdevalence]

(It may be easier in the short term to use Serde's remote derive functionality, https://serde.rs/remote-derive.html , so that this doesn't block on upstream.)

[chelseakomlo]

Closing this as we merged #47 ; will open a separate issue for the DKG.