Follow-up works for Bn254.sol

[alxiong]

To avoid a overly bulky PR, we leave the following future works on BN254.sol on top of #59:

• add expect(ContractCall).to.revertWithMessage() to test validateG1Point function
• remove From<(U256, U256)> for G1Point since ethers.rs added return struct support: Add return struct support from abigen function signatures gakonst/ethers-rs#661
• add field validation utility functions (might be used for asset code validation later)
• maybe rename library Curve -> library BN254? (done in refactor: add contract folder struct #71 )