[tarcieri]

It seems I made a mistake extracting these into ctutils. I thought that might be a nice place to handle modeling borrows/overflows as Choice but in the course of both trying to fix the constant-time issues in crypto-bigint and then try to restore some of the previous performance I discovered the actual requirements of the function changed because I accidentally shipped a function signature with a performance bottleneck, and then in the course of subsequent refactoring I found I wasn't even able to salvage overflowing_add_*.

So I am declaring the entire ctutils::primitives module a mistake, but it's already published, so the best I can do is remove crypto-bigint's dependence on it and then deprecate it.

This moves the exact primitives that crypto-bigint needs back to the crypto_bigint::primitives module, which uses intrinsics we know work when available with a software fallback.

It also adds cargo checkct tests for these primitives.

[codecov]

Codecov Report

❌ Patch coverage is 85.29412% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.37%. Comparing base (ec04d14) to head (f87f8cb).

Files with missing lines	Patch %	Lines
src/primitives.rs	84.37%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1377      +/-   ##
==========================================
- Coverage   91.39%   91.37%   -0.02%     
==========================================
  Files         190      190              
  Lines       23128    23192      +64     
==========================================
+ Hits        21138    21192      +54     
- Misses       1990     2000      +10     

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