add CompareTwoPairings

[herumi]

The Verify function computes e(P1, Q1) == e(P2, Q2) where e is pairing. But Pairing(P, Q) = FE(ML(P, Q)) where ML is MillerLoop and FE is FinalExponentiation. e(P1, Q1) == e(P2, Q2) <=> e(P1, Q1) e(-P2, Q2) == 1 <=> FE(ML(P1, Q1) ML(P2, Q2)) == 1 then I added CompareTwoPairings, which can reduce one FE. I got the following speed on Core i7-7700 3.6GHz:

• original g1pubs

  bls/g1pubs% go test --bench .
  goos: linux
  goarch: amd64
  pkg: github.com/phoreproject/bls/g1pubs
  BenchmarkBLSAggregateSignature-4                  200000              8234 ns/op
  BenchmarkBLSSign-4                                   500           3755491 ns/op
  BenchmarkBLSVerify-4                                 100          14766535 ns/op
  BenchmarkVerifyWithDomain-4                          100          16862432 ns/op
  BenchmarkVerifyAggregateCommonWithDomain-4           100          16845176 ns/op
  BenchmarkVerifyAggregateMultipleWithDomain-4           1        1327194394 ns/op

• optimized g1pubs

  pkg: github.com/phoreproject/bls/g1pubs
  BenchmarkBLSAggregateSignature-4                  200000              8189 ns/op
  BenchmarkBLSSign-4                                   500           3809514 ns/op
  BenchmarkBLSVerify-4                                 100          10010070 ns/op
  BenchmarkVerifyWithDomain-4                          100          11823728 ns/op
  BenchmarkVerifyAggregateCommonWithDomain-4           100          11900917 ns/op
  BenchmarkVerifyAggregateMultipleWithDomain-4           1        1349163898 ns/op

• original g2pubs

  bls/g2pubs% go test --bench .
  BenchmarkBLSAggregateSignature-4         1000000              2143 ns/op
  BenchmarkBLSSign-4                          2000           1073271 ns/op
  BenchmarkBLSVerify-4                         100          13116371 ns/op

• optimized g2pubs

  goos: linux
  goarch: amd64
  pkg: github.com/phoreproject/bls/g2pubs
  BenchmarkBLSAggregateSignature-4         1000000              2144 ns/op
  BenchmarkBLSSign-4                          2000           1081131 ns/op
  BenchmarkBLSVerify-4                         200           8058618 ns/op

[meyer9]

Thank you for the contribution! Merged.