[WIP] Add pairings over 256-bit Barreto Naehrig curve

[randombit]

• [ ] Provide an abstract interface that can be implemented across several pairing types (eg BLS or BN-448). TBH I'm not even sure how to approach this in C++ given the need for several sub classes (G1,G2,GT) which are associated with the pairing. But it would be useful to implement things like BF-IBE or BLS signatures just once and then use them across multiple pairing types.
• [x] Should use Montgomery_Int from #1461 instead of GFp1
• [ ] Add hashing on the curve (APIs are there but not implemented)
• [ ] Documentation
• [ ] More tests (going to fuzz vs dclxvi implementation to generate test vectors and provide more assurance of correctness)
• [ ] Demo, maybe BLS short signatures

[codecov-io]

Codecov Report

Merging #1432 into master will increase coverage by 0.02%. The diff coverage is 87.83%.

@@            Coverage Diff             @@
##           master    #1432      +/-   ##
==========================================
+ Coverage   92.03%   92.05%   +0.02%     
==========================================
  Files         517      519       +2     
  Lines       55140    55839     +699     
  Branches     5740     5754      +14     
==========================================
+ Hits        50748    51403     +655     
- Misses       4392     4436      +44

Impacted Files	Coverage Δ
src/tests/test_bn256.cpp	100% <100%> (ø)
src/cli/speed.cpp	87.92% <3.22%> (-2.83%)	:arrow_down:
src/lib/pairings/bn256/bn256.cpp	91.24% <91.24%> (ø)
src/lib/math/bigint/big_ops2.cpp	96.47% <0%> (+0.7%)	:arrow_up:
src/lib/pubkey/mce/mceliece_key.cpp	86.51% <0%> (+1.12%)	:arrow_up:
src/lib/misc/cryptobox/cryptobox.cpp	95.23% <0%> (+1.58%)	:arrow_up:
src/lib/math/numbertheory/monty.cpp	81.46% <0%> (+15.94%)	:arrow_up:

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