fix!: eliminate unnecessary merkle proofs in ADVZ shares

[ggutoski]

closes: #671

This PR:

• Advz::evals_proof is no longer a Vec
• structure of the merkle tree is changed accordingly. Each leaf in the merkle tree is a vec of polynomial evaluations as before, except now that vec looks like: https://github.com/EspressoSystems/jellyfish/blob/689555e07860c5ed6e54b467c76d9a1776f0f7b5/vid/src/advz.rs#L702-L715
• delete test sad_path_verify_share_with_multiplicity as it is a regression for #654 , which is no longer relevant as there is now only one merkle path in each share.

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