Verify or replace G2 coefficient value

[FiloSottile]

The G2 coefficient value, used to scale in hash_to_g2, was copied verbatim from the Rust implementation. It should be checked, or replaced with a RELIC function.

powersoftau/bls12/g2.c

Line 33 in ce98e03

bn_read_str(k, "5d543a95414e7f1091d50792876a202cd91de4547085abaa68a205b2e5a7ddfa628f1cb4d9e82ef21537e293a6691ae1616ec6e786f0c70cf1c38e31c7238e5", 127, 16); // FROM RUST IMPLEMENTATION // TODO: verify

Upstream issue: relic-toolkit/relic#64

[FiloSottile]

Verified the value with a script adapted from relic-toolkit/relic#55 (comment)

https://github.com/FiloSottile/powersoftau/blob/f8d564b0/bls12/parameters.sage