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:

  jellyfish/vid/src/advz.rs

  Lines 702 to 715 in 689555e

  	// Populate a Vec of polynomial evaluations for all storage nodes:
  	//
  	// The `i`th item is a Vec of `polys.len() * multiplicity`
  	// polynomial evaluations.
  	//
  	// Evaluations for storage node `i` are ordered as follows. Define
  	// - `n`: the number of storage nodes: `self.num_storage_nodes`
  	// - `k`: the number of polynomials minus 1: `polys.len() - 1`
  	// - `m`: `multiplicity - 1`
  	// - `p[j](x)`: the value of the `j`th polynomial evaluated at `x`.
  	//
  	// p[0](i), p[0](i+n), ..., p[0](i+m*n),
  	// ...,
  	// p[k](i), p[k](i+n), ..., p[k](i+m*n),

• delete test sad_path_verify_share_with_multiplicity as it is a regression for VID: verify consistency with the merkle root for all the evaluations in a VID share #654 , which is no longer relevant as there is now only one merkle path in each share.

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[akonring]

Nice.
Don't fully understand the motivation for moving to a different "share evaluation layout".
But they seem equivalent and the docs added are great and needed. Thanks.