Stake table contract improvements

[philippecamacho]

• Implement ParsedG2Point struct (done in solidity-bn254 crate)
• emit event EpochChanged so that the prover service can subscribe the even stream and know when to pull new stake table snapshot from L1
• Update to the latest version of BN254.sol (already cover by another issue)
• Delete HotShot.sol contract which is obsolete (Done as part of Update bn254 contract dependency #775)
• Use testFuzz to ensure the BLS key is validated correctly. See Implementation of the Stake table contract (register function) #769 (review)
• Add invariant test relative to the balances. See Implementation of the Stake table contract (register function) #769 (comment)

[philippecamacho]

Blocked by #775.