perf: direct MillerLoop in 2-chains precomputes G2 lines on copies

[ivokub]

Overflow issue from #949:

for two-chain we have wrapper for MillerLoop which goes from []G2Affine argument to []*G2Affine. But we compute the lines in the method which uses []G2Affine. However, as the slice contains of values then inside a method we are working on a copy of G2Affine value and we are not updating the lazy lines of the input. I think we can either change MillerLoop to take []*G2Affine so that can modify inline or precompute the lines in Pairing wrapper method Pairing.MillerLoop to already precompute. I like the first approach better because then we always will use lazy line computation.

But I guess better to resolve with #863