More efficient SolidityTranscript

[alxiong]

Based on the suggestion from CommonPrefix,

we could:

• use big endian serialization (instead of the LE default from arkworks), so that we avoid reversing them in solidity contract
• avoid prepend the state in computation of the next challenge: directly use H(transcripts) instead of H( state | transcript )

These changes are necessary before EspressoSystems/espresso-sequencer#1640 can be implemented.