Low-level refactor part 2

[mratsim]

This builds on top of #175 and closes #133

This accelerates reduces BLS12-381 square root computations cost by ~13.5%

• Before
  
• After
  

Those are a bottleneck in:

• Signature and Public key decompression (and in Ethereum everything comes from the network compressed), clean up / document init status-im/nimbus-eth2#3387 (comment)
• hashing a message to the curve for its signature and verification.
  • Hence people are optimizing the heck out of it in Solidity: https://github.com/ChihChengLiang/modexp, used in this optimistic rollup project https://github.com/thehubbleproject/hubble-contracts/blob/cc3aa97d3d47b30516ad73077a8f6e8c6e834bab/contracts/libs/ModExp.sol#L332-L340
• test suite! Creating any random point for fuzzing requires either sqrt or the 5x more expensive scalar multiplication. And tests already take a while ....