crypto: Optimize bit test of scalar in ECC multiplication

[chfast]

Add a separate procedure for testing a bit in uint256. It improves readability and performance slightly (up to 1%).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.25%. Comparing base (92e8882) to head (9720a1d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1336   +/-   ##
=======================================
  Coverage   87.24%   87.25%           
=======================================
  Files         169      169           
  Lines       24970    24977    +7     
  Branches     4106     4106           
=======================================
+ Hits        21786    21793    +7     
  Misses        529      529           
  Partials     2655     2655           

Flag	Coverage Δ
eest-develop	79.89% <100.00%> (+0.01%)	⬆️
eest-develop-gmp	15.96% <0.00%> (-0.01%)	⬇️
eest-legacy	11.21% <0.00%> (-0.01%)	⬇️
eest-legacy-silkpre	17.65% <87.50%> (+0.02%)	⬆️
evmone-unittests	83.82% <100.00%> (+<0.01%)	⬆️

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Components	Coverage Δ
core	93.59% <100.00%> (+<0.01%)	⬆️
tooling	88.08% <ø> (ø)
tests	84.14% <ø> (ø)

Files with missing lines	Coverage Δ
lib/evmone_precompiles/ecc.hpp	96.61% <100.00%> (+0.10%)	⬆️

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[rodiazet]

LGTM