use u128 for bigint limbs

[andresilva]

This PR updates the bigints (U512 and U256) to use u128 for limbs.
The fields' inverses were defined over 2^64 so I had to extend them to 2^128.

benchcmp

name                         control ns/iter  variable ns/iter  diff ns/iter   diff %  speedup
 fq12_exponentiation          4,544,644        2,338,371           -2,206,273  -48.55%   x 1.94
 fq12_scalar_multiplication   14,569           7,125                   -7,444  -51.09%   x 2.04
 fr_addition                  17               13                          -4  -23.53%   x 1.31
 fr_inverses                  8,711            5,531                   -3,180  -36.51%   x 1.57
 fr_multiplication            104              41                         -63  -60.58%   x 2.54
 fr_subtraction               20               14                          -6  -30.00%   x 1.43
 g1_addition                  1,805            756                     -1,049  -58.12%   x 2.39
 g1_deserialization           768              482                       -286  -37.24%   x 1.59
 g1_scalar_multiplication     481,975          212,330               -269,645  -55.95%   x 2.27
 g1_serialization             9,898            6,428                   -3,470  -35.06%   x 1.54
 g1_serialization_normalized  549              431                       -118  -21.49%   x 1.27
 g1_subtraction               1,872            771                     -1,101  -58.81%   x 2.43
 g2_addition                  8,440            3,768                   -4,672  -55.36%   x 2.24
 g2_deserialization           1,776,474        864,284               -912,190  -51.35%   x 2.06
 g2_scalar_multiplication     2,056,874        983,488             -1,073,386  -52.19%   x 2.09
 g2_serialization             12,477           7,628                   -4,849  -38.86%   x 1.64
 g2_serialization_normalized  1,039            752                       -287  -27.62%   x 1.38
 g2_subtraction               8,388            3,820                   -4,568  -54.46%   x 2.20
 perform_pairing              5,315,023        2,644,591           -2,670,432  -50.24%   x 2.01

[NikVolf]

why renaming?

[andresilva]

It conflicts with the From implementation I added. I can remove the impl From<[u64; 8]> for U512 and just add it directly to U512 to avoid changing the name of this method.

[NikVolf]

Ah, ok then

[NikVolf]

Great job, though i'd prefer not to intruduce unnecessary changes from upstream