Add support for BLS12-381

[jannotti]

Tentative support for bn256 was recently added in #4013 . BLS12-381 should also be supported. Further, there should be support for (at least) the nine opcodes of https://eips.ethereum.org/EIPS/eip-2537 rather than just the three done for bn256.

The extra 6 opcodes from eip 2537 ought to be done bn256 too, just for parity.

There is an additional opcode beyond EIP 2537 that is also likely to be valuable. @fabrice102 can probably remember what it is an comment.

Implementation should be done with gnark-crypto the same library used for bn256, which is in the process of undergoing audit.

[fabrice102]

We were thinking whether we want to include operations in GT.

One main drawback of including such operations is that it forces to select a representation of GT and prevents any easy change. In addition, it would mean that the product of pairing operation need not checking equality to 1 but actually returning the value of the pairing.
In Groth16, multiplication in GT would remove the need of one pairing. It is unclear how much of a benefit it is in practice, since pairings are amortized.

Another question is whether to explicitly provide scalar group operations. Those operations can easily be done with current opcode (b+, b*, b%) but it may be cleaner to have explicit opcode for those.

[jannotti]

This is merged (though still in vFuture)