Improve performance of Monty pow/pow_vartime

[andrewwhitehead]

This is mostly a simple change to access the elements of powers_and_exponents by reference, which improves performance by 2-3% in my tests.

The constant WINDOW_COUNT is added to resolve an error displayed by rust-analyzer due to (I assume) the equality of evaluated constants not being guaranteed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.96%. Comparing base (3c38f48) to head (15aaf0e).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1222   +/-   ##
=======================================
  Coverage   87.96%   87.96%           
=======================================
  Files         184      184           
  Lines       21172    21172           
=======================================
  Hits        18624    18624           
  Misses       2548     2548           

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[andrewwhitehead]

It also looks like primefield might benefit from using this pow_vartime for performance