mastic: Add test vectors, fix remaining bugs

Add test vectors for Mastic and fix the bugs they encounter:

* Add domain separation to the payload check hash.
* Truncate the weights prior to aggregation.
* Adjust the order of the binder parts in the eval proof computation.
* vidpf: Align public share encoding with the draft.
* vidpf: Adjust index encoding in node proof.
* vidpf: Align a few variable names with the spec.

The test vectors were generated after making some changes to the spec
based on our implementation experience here:
jimouris/draft-mouris-cfrg-mastic#101

src/flp/szk.rs

@@ -202,7 +202,7 @@ impl<F: FieldElement + Decode, const SEED_SIZE: usize> ParameterizedDecode<(bool
 }
 
 /// A tuple containing the state and messages produced by an SZK query.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, PartialEq)]
 pub struct SzkQueryShare<F: FieldElement, const SEED_SIZE: usize> {
     joint_rand_part_opt: Option<Seed<SEED_SIZE>>,
     pub(crate) flp_verifier: Vec<F>,