Move u128 conversions out of Field trait

[andyleiserson]

There are a lot of changed files, but most of them are only changed to update references to Field or U128Conversions traits. The core change here is that the truncate_from and as_u128 methods are removed from the Field trait and put in the new U128Conversions trait, because the u128 conversions do not make sense on Fp25519.

There are some more substantial upcoming changes that will resolve the remaining issues with the Field trait and clean up the TODOs in this PR.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (860e046) 88.44% compared to head (8433d08) 88.43%.

Files	Patch %	Lines
ipa-core/src/test_fixture/mod.rs	20.00%	4 Missing ⚠️
ipa-core/src/test_fixture/circuit.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #954      +/-   ##
==========================================
- Coverage   88.44%   88.43%   -0.01%     
==========================================
  Files         158      158              
  Lines       21472    21472              
==========================================
- Hits        18991    18989       -2     
- Misses       2481     2483       +2     

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[akoshelev]

oof merging it with ipa v2 removal change will be fun