Include Add<&Self> in SharedValue

[andyleiserson]

This will be required to eliminate Add bounds from the shuffle traits.

This is on top of #1394.

[codecov]

Codecov Report

Attention: Patch coverage is 88.70968% with 42 lines in your changes missing coverage. Please review.

Project coverage is 93.64%. Comparing base (b8a5fd1) to head (e1adf57).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
ipa-core/src/protocol/ipa_prf/shuffle/mod.rs	15.38%	22 Missing ⚠️
ipa-core/src/protocol/context/malicious.rs	0.00%	12 Missing ⚠️
ipa-core/src/ff/curve_points.rs	50.00%	4 Missing ⚠️
ipa-core/src/protocol/ipa_prf/shuffle/malicious.rs	98.27%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1395      +/-   ##
==========================================
- Coverage   93.68%   93.64%   -0.04%     
==========================================
  Files         223      223              
  Lines       37737    37976     +239     
==========================================
+ Hits        35353    35562     +209     
- Misses       2384     2414      +30     

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[andyleiserson]

We actually don't have to do this, if we believe that Copy is always cheap. Shuffle operates on BooleanArrays, which are SharedValues, which are Copy. (Originally, ShuffleShare only required Sendable + Clone + FromRandom + Add<Output = Self>. That's not sufficient for the malicious shuffle MACs. But we could come up with something less than BooleanArray.)

For the things that we currently shuffle, I think it's reasonable to assume Copy is cheap. But I'm not sure we want to bake that assumption in to the implementation, which otherwise could easily be used to shuffle larger things.

We may also want to remove the implication that BooleanArrays are Copy if we add larger boolean arrays to increase vectorization dimensions for performance.

[andyleiserson]

I don't think we need this. I am going to use a more precise set of bounds for ShuffleShare.