Store commitments in blob

[martyall]

Summary

This PR changes the way we (de)serialize blobs to use macros rather than doing it manually -- there should be no net effect. I also started storing the commitments in the blob metadata to save us from having to recompute them for reads.

Finally there is a small change to the "sanity check" to make sure the user and storage provider agree on the data. This is not strictly part of the protocol, but rather it helps with the testing.

Changes

• use the serde_as package to derive the serialization instances: previously defined serialization instances by hand using arkworks CanonicalSerialize trait. I found other places in the codebase that used this simpler approach.
• store the commitments in the FieldBlob struct: otherwise we will need to recompute them for each read request. This means FieldBlob is now parameterized by the group instead of the scalar field.
• stop returning the "randomized sum of commitments" from store request -- they are not used until later anyway. We can just return a hash for now for the "sanity check". @marcbeunardeau88 Now I understand this is what you were saying here

[codecov]

Codecov Report

Attention: Patch coverage is 60.60606% with 13 lines in your changes missing coverage. Please review.

Project coverage is 76.82%. Comparing base (e0ac06d) to head (8128e3d).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
saffron/src/main.rs	0.00%	12 Missing ⚠️
saffron/src/commitment.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2967   +/-   ##
=======================================
  Coverage   76.81%   76.82%           
=======================================
  Files         260      260           
  Lines       61371    61307   -64     
=======================================
- Hits        47145    47098   -47     
+ Misses      14226    14209   -17     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[martyall]

Not deleted -- this and the tests below just got moved to the blob module

[Fizzixnerd]

looks good