𝔾ₜ exponentiation, with endomorphism acceleration

[mratsim]

This PR adds a performance baseline for Whisk in GT.

https://ethresear.ch/t/the-return-of-torus-based-cryptography-whisk-and-curdleproof-in-the-target-group/16678

Here is the speed on a 7840U:

For comparison here is scalar mul on G1

and on G2

Hence exponentiation in Gt (131 us) is within 30% of scalar mul in G2 (96 us).

cc @asanso

[mratsim]

Gnark bench

cd ecc/bls12-381
go test -bench=ExpGT -run=none --cpu 1

so 2.29x faster than Gnark.