feat: bls12-381 support

[chancehudson]

Closes #203

This PR implements zkey loading, proving, and verifying over a generic curve. We automatically detect the curve from the zkey and use the correct implementation, or error if using an unrecognized curve. Supported curves:

• alt_bn128
• bls12_381

We should check binary sizes when adding additional curves. It might be worth conditionally compiling the curves.

TODO:

• add test bls zkey
• load groth header with manual Fq/Fr parsing, no field type

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[chancehudson]

This PR is ready for review. I wrote a poseidon implementation for bls and wanted to use that for the tests here but it's making incorrect proofs. I documented the issue in #208 and added a different circuit for testing bls.

[vivianjeng]

Good PR!
I tried on iOS simulator and it works for me
I think we can add some e2e tests
and update bindings tests
(I can help with these)