VID share distribution algorithm (2)

[rob-maron]

Closes #2104

This PR:

• Changes the number of shares and chunk size to:
  • Conform to VID changes (power of 2)
  • Use the quorum membership
• Adds a constructor for VidDisperse that takes a membership for share distribution
• Implements an algorithm for the constructor to deterministically disperse VID shares based on index
• Changes the testing function vid_init to use the constructor so it's easy to use during tests
• Edits the tests to conform to this

This PR does not:

• Change any other VID functionality
• Address networking or actual distribution of shares

Key places to review:

vid.rs - where we do the actual dispersal
data.rs - where we specify what algorithm to use for dispersal given membership and the shares

This PR augments and fixes #2149 by modifying the vid_commitment function. All the relevant (unapproved) changes are in the second commit "second set of changes" and the commit "remove quorum membership from DA".

[ggutoski]

There's a couple design questions we need to discuss sometime. Not sure whether we need to resolve these questions before merging this PR.

[ggutoski]

It's a thing of beauty!

The base branch was changed.

[shenkeyao]

Looks good! Thanks for the patience for being back and forth regarding the quorum membership.