Offer a multi-DLEq proof which simply merges challenges for n underlying proofs

This converts proofs from 2n elements to 1+n.

Moves FROST over to it. Additionally, for FROST's binomial nonces, provides
a single DLEq proof (2, not 1+2 elements) by proving the discrete log equality
of their aggregate (with an appropriate binding factor). This may be split back
up depending on later commentary...

crypto/frost/src/tests/vectors.rs

@@ -182,8 +182,8 @@ pub fn test_with_vectors<R: RngCore + CryptoRng, C: Curve, H: Hram<C>>(
           commitments: Commitments {
             nonces: vec![NonceCommitments {
               generators: vec![GeneratorCommitments(these_commitments)],
-              dleqs: None,
             }],
+            dleq: None,
           },
           addendum: (),
         },