crypto: Use variadic-length modinv in modexp

[chfast]

Replace the modular inversion with mod 2ᵏ which uses fixed size intx::uint to one which uses variadic-length numbers represented by std::span<uint64_t>.
This improves performance in pathological cases when the 2ᵏ part of the modexp modulus is significant shorter than the modulus itself.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (2c62685) to head (5dc11af).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1434   +/-   ##
=======================================
  Coverage   96.51%   96.52%           
=======================================
  Files         152      152           
  Lines       13805    13828   +23     
  Branches     3223     3225    +2     
=======================================
+ Hits        13324    13347   +23     
  Misses        342      342           
  Partials      139      139           

Flag	Coverage Δ
eest-develop	92.09% <100.00%> (+0.04%)	⬆️
eest-develop-gmp	29.15% <100.00%> (+0.12%)	⬆️
eest-legacy	15.33% <0.00%> (-0.03%)	⬇️
eest-legacy-silkpre	24.66% <100.00%> (+0.12%)	⬆️
evmone-unittests	91.25% <100.00%> (+0.01%)	⬆️

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Components	Coverage Δ
core	94.91% <100.00%> (+0.02%)	⬆️
tooling	85.28% <ø> (ø)
tests	99.77% <ø> (ø)

Files with missing lines	Coverage Δ
lib/evmone_precompiles/modexp.cpp	100.00% <100.00%> (ø)

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[Copilot]

Pull request overview

This pull request refactors the modular inversion function in the modular exponentiation implementation to use variable-length operands via spans instead of fixed-size template types.

Changes:

• Adds two new helper functions: mul() for truncated multiplication and neg_add2() for computing 2 - x
• Converts modinv_pow2() from a template function to a span-based function for variable-length operations
• Updates the call site in modexp_even() to use the new span-based interface

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

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Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

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