Refactor/bw6-761-pairing

[yelhousni]

• Miller loop according to ABLR13 (same as the other curves).
• Fp6 as a quadratic extension over Fp3 (instead of a cubic extension over Fp2).

benchcmp vs develop

• pairing

BenchmarkPairing-2          3080057       2788557       -9.46%
BenchmarkMultiPairing-2     5761097       4970379       -13.73%

• Miller loop

benchmark                 old ns/op     new ns/op     delta
BenchmarkMillerLoop-2     1978253       1634283       -17.39%

• final exp

benchmark                          old ns/op     new ns/op     delta
BenchmarkFinalExponentiation-2     1107366       1123309       +1.44%

• Fp6

benchmark                    old ns/op     new ns/op     delta
BenchmarkE6Add-2             77.4          75.6          -2.33%
BenchmarkE6Sub-2             59.4          55.2          -7.07%
BenchmarkE6Mul-2             3001          2964          -1.23%
BenchmarkE6Cyclosquare-2     1424          1444          +1.40%
BenchmarkE6Square-2          2120          2123          +0.14%
BenchmarkE6Inverse-2         26141         25171         -3.71%
BenchmarkE6Conjugate-2       54.3          19.7          -63.72%
BenchmarkE6Frobenius-2       485           573           +18.14%
BenchmarkE6Expt-2            107347        106262        -1.01%