Optimize BW6 final exponentiation

[yelhousni]

Optimize the final exponentiation (FE) of BW6 curves following algorithms 4.3, 4.4, 4.5 and 4.6 of https://yelhousni.github.io/phd.pdf. The PR Consensys/gnark#846 uses alg. 4.4 in-circuit, and for testing the hack now is to raise the result to 12. In fact, currently in gnark-crypto the FE power is 12(x₀+1)(p⁶-1)/r and in gnark (x₀+1)(p⁶-1)/r. Implementing these algorithms should speedup the different BW6 pairings in gnark-crypto too.