feat: multiplicity depend on payload size

[ggutoski]

closes: #663

This PR:

This PR replaces #667. Thanks to @akonring for getting this PR started.

The fix for issue #663 has many knock-on tasks, most of which are also resolved here.

• multiplicity arg for Advz constructor is now max_multiplicity. The user no longer specifies the multiplicity that is used for all payloads. Instead, the user specifies the maximum multiplicity that may be used. The actual multiplicity is now selected at disperse-time as a function of payload size. Thus, as per Fix large shares when small payload and large multiplicity #663, a small payload with large multiplicity will no longer have wastefully-large share size.
• new test max_multiplicity that ensures small multiplicity for small payloads
• As noted in feat: let multiplicity depend on payload size #667 (comment), it is insufficient merely to set multiplicity at disperse-time. We must also set the size of the FFT domains at disperse-time.
• Prior to this PR those domains were stored in fields eval_domain, multi_open_domain of the Advz struct. Now that these domains are set at disperse-time there is no need to store them in the Advz struct, so I removed them. They are now computed on-the-fly as needed, just like multiplicity.
• Implementing these changes was tedious. It became clear to me that we have some tech debt. Thus, I also did some code clean-up in this PR. This clean-up did not cost much extra time because I had to do 90% of the work anyway just to implement the changes needed for this PR.
• Tech debt:
  • disperse_precompute is mostly copy-pasted code from disperse. I refactored these two functions into disperse_with_polys_and_commits.
  • Lots of low-level helpers were messy: polynomial, polynomial_internal, bytes_to_polys, etc
• Posted new issue ADVZ: drop the restriction that recovery_threshold, multiplicity must be powers of two #668 and linked to it in several places in code comments.

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[akonring]

Nice!
Made a few comments (mostly nits so feel free to ignore if they are out of scope or don't apply).

[akonring]

LGTM.