[WIP] Handle power-of-two moduli in mul_mod_special

[tarcieri]

Alternative to #1282 which aims to preserve constant-time operation when handling this case.

Closes #1281

[tarcieri]

Added tests but I'd still like to gauge the impact on benchmarks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (d1a4e1c) to head (7bad5f0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1352   +/-   ##
=======================================
  Coverage   91.28%   91.29%           
=======================================
  Files         190      190           
  Lines       22941    22963   +22     
=======================================
+ Hits        20942    20964   +22     
  Misses       1999     1999           

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[tarcieri]

Some notable benchmark changes:

wrapping ops/boxed_rem_limb
                        time:   [448.58 ns 452.99 ns 457.71 ns]
                        change: [+23.212% +26.499% +29.870%] (p = 0.00 < 0.05)
                        Performance has regressed.

Const Montgomery arithmetic/multi_exponentiate for 10 bases, U256^U256
                        time:   [32.855 µs 33.119 µs 33.431 µs]
                        change: [+7.8154% +12.499% +17.870%] (p = 0.00 < 0.05)
                        Performance has regressed.