feat: use Halo2 compatible dependencies

[DrPeterVanNostrand]

• Switches dependencies to ff, group, and pairing
• Adds the type Gt which is a prime order subgroup of Fp12; output by Fp12::final_exponentiation() and Bls12::pairing()
• Implement ff::PrimeFieldBits for Scalar

[codecov-commenter]

Codecov Report

Merging #22 (285276b) into master (485e19a) will decrease coverage by 2.50%. The diff coverage is 85.41%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   86.81%   84.31%   -2.51%     
==========================================
  Files          15       14       -1     
  Lines        6819     6688     -131     
==========================================
- Hits         5920     5639     -281     
- Misses        899     1049     +150     

Impacted Files	Coverage Δ
src/pairing.rs	12.28% <22.58%> (-0.96%)	:arrow_down:
src/macros.rs	57.14% <47.36%> (+3.29%)	:arrow_up:
src/g2.rs	74.00% <67.71%> (-4.27%)	:arrow_down:
src/g1.rs	76.56% <73.96%> (-5.20%)	:arrow_down:
src/gt.rs	85.11% <85.11%> (ø)
src/fp12.rs	79.66% <86.01%> (+2.37%)	:arrow_up:
src/serde_impl.rs	93.72% <86.11%> (-5.02%)	:arrow_down:
src/fp6.rs	82.96% <87.64%> (-10.56%)	:arrow_down:
src/fp.rs	86.68% <92.46%> (-1.01%)	:arrow_down:
src/scalar.rs	90.49% <94.39%> (+2.05%)	:arrow_up:
... and 15 more

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