modexp: Skip the leading zeros in the exponent

[chfast]

Introduce the Exponent view type to represent the exponent bytes as an array of bits. Skip the leading exponent zero bytes and bits.

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.25%. Comparing base (ae53163) to head (84d8a24).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/evmone_precompiles/modexp.cpp	96.87%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1292   +/-   ##
=======================================
  Coverage   87.25%   87.25%           
=======================================
  Files         169      169           
  Lines       24976    24984    +8     
  Branches     4107     4106    -1     
=======================================
+ Hits        21792    21800    +8     
  Misses        529      529           
  Partials     2655     2655           

Flag	Coverage Δ
eest-develop	79.94% <93.75%> (+0.03%)	⬆️
eest-develop-gmp	16.00% <75.00%> (+0.03%)	⬆️
eest-legacy	11.21% <0.00%> (-0.01%)	⬇️
eest-legacy-silkpre	17.69% <78.12%> (+0.03%)	⬆️
evmone-unittests	83.83% <84.37%> (+0.01%)	⬆️

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

Components	Coverage Δ
core	93.59% <96.87%> (+<0.01%)	⬆️
tooling	88.08% <ø> (ø)
tests	84.14% <ø> (ø)

Files with missing lines	Coverage Δ
lib/evmone_precompiles/modexp.cpp	95.79% <96.87%> (+0.30%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.