Solidity gas optimizations

Figure out where we can cut gas by optimizing the UpdateClient and ReceivePacket handlers.

For the former, we currently apply:

[x] smallest possible set for the public input of the ZK circuit
[x] ClientState using ETH ABI instead of Protobuf
[x] optimized, evm slot-friendly ConsensusState structure using ETH ABI insteand of Protobuf
[x] inline merkle root for block header

More optimizations (probably v2):

move everything in the ZKP circuit (would require non-trivial upgrade of the circuit, not sure it is worth the effort)
use ZK friendly hashing like MiMC in place of sha256 (require the above to be implemented first)

For packets, the relayer could bulk them and provide a ZKP of aggregated membership proof verification (circuit yet to be done).

unionlabs / union