Fix logic gate

[moCello]

The assert at the end of the logic_gate function did not pass for bit_num = 256. The problem was in the unfortunate fact that the BlsScalar created from the bit_mask = [u64::MAX, u64::MAX, u64::MAX, u64::MAX] is in fact invalid. It seems that all BlsScalars created with the from_raw function are invalid if the raw value is larger than the MODULO.

[codecov]

Codecov Report

Merging #679 (4d4e73c) into master (4a672c5) will decrease coverage by 0.01%.
The diff coverage is 93.84%.

@@            Coverage Diff             @@
##           master     #679      +/-   ##
==========================================
- Coverage   82.05%   82.03%   -0.02%     
==========================================
  Files          54       54              
  Lines        4418     4437      +19     
==========================================
+ Hits         3625     3640      +15     
- Misses        793      797       +4     

Impacted Files	Coverage Δ
src/constraint_system/logic.rs	93.07% <93.84%> (-2.42%)	⬇️
src/plonkup/multiset.rs	73.62% <0.00%> (-0.55%)	⬇️
src/proof_system/prover.rs	88.44% <0.00%> (-0.33%)	⬇️
src/permutation.rs	90.00% <0.00%> (+0.21%)	⬆️
src/fft/polynomial.rs	41.32% <0.00%> (+0.82%)	⬆️

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