Eliminate unnecessary merkle proofs in ADVZ shares

[ggutoski]

This issue drawn from #657 (comment)

Currently each share now has multiplicity distinct merkle paths instead of just 1. That's a waste of bandwidth and it breaks backwards compatibility with jellyfish tag 0.4.5.

We should have only a single merkle proof for all evals.

[ggutoski]

The fix #657 breaks backwards compatibility with jellyfish tag 0.4.5 for serialization of ADVZ's Share struct because this field

jellyfish/vid/src/advz.rs

Line 313 in 7d71dbe

evals_proof: KzgEvalsMerkleTreeProof<E, H>,

...became this

jellyfish/vid/src/advz.rs

Line 314 in 7cd4f76

eval_proofs: Vec<KzgEvalsMerkleTreeProof<E, H>>,

It seems impossible restore backwards compatibility. Any fix for this issue that restores evals_proof to its original type must change how the merkle tree is built, in which case future versions of jf-vid will disagree with past versions on how to build the merkle tree.

Given that we must break backwards compatibility somewhere, we should fix this issue and break how the merkle tree is built. The only alternative is to not fix this issue, in which case we continue to unnecessarily bloat share size by including unneeded merkle paths.