modexp: Optimize the exponentiation loop

[chfast]

Use the information that in the first iteration of the exponentiation loop the bit is aways 1.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.33%. Comparing base (37eed6f) to head (aef2d09).
⚠️ Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/evmone_precompiles/modexp.cpp	60.00%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1367      +/-   ##
==========================================
- Coverage   82.55%   81.33%   -1.23%     
==========================================
  Files         152      152              
  Lines       13423    13429       +6     
  Branches     3215     3217       +2     
==========================================
- Hits        11082    10923     -159     
- Misses        194      294     +100     
- Partials     2147     2212      +65     

Flag	Coverage Δ
eest-develop	?
eest-develop-gmp	23.27% <60.00%> (+0.80%)	⬆️
eest-legacy	15.49% <0.00%> (-0.01%)	⬇️
eest-legacy-silkpre	24.80% <60.00%> (+0.01%)	⬆️
evmone-unittests	75.95% <60.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
core	94.61% <60.00%> (-2.76%)	⬇️
tooling	79.58% <ø> (-2.57%)	⬇️
tests	73.14% <ø> (ø)

Files with missing lines	Coverage Δ
lib/evmone_precompiles/modexp.cpp	87.64% <60.00%> (-6.34%)	⬇️

... and 17 files with indirect coverage changes

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