feat: optimize Godwoken finality mechanism

[keroro520]

Motivation & Overview

https://talk.nervos.org/t/optimize-godwoken-finality-and-on-chain-cost/6739

Implementation Note

Summary

The finality mechanism determines whether an entity has been finalized or not.

Entities that require finality checking must contain an attribution indicating their occuration time, which can be found in godwoken.mol:

Entity	Occuration Attribution
custodian_state_cell	deposit_block_number
withdrawal_state_cell	withdrawal_block_number
stake_state_cell	stake_block_number

Convention

In order to denote entities' occurrences, we define a compatible type, Timepoint:

// Layout of Timepoint:
//   - the highest 1 bit represent whether the time point is block-number-based
//     or timestamp-based
//   - the rest 63 bits represent the value of time point
pub enum Timepoint {
	BlockNumber(/* block_number: */ u64),
	Timestamp(/* timestamp: */ u64),
}

impl Timepoint {
    const MASK: u64 = 1 << 63;

    pub fn from_full_value(full_value: u64) -> Self {
        let is_block_number = (Self::MASK & full_value) == 0;
        if is_block_number {
            Timepoint::BlockNumber(full_value)
        } else {
            Timepoint::Timestamp(Self::MASK ^ full_value)
        }
    }

    pub fn full_value(&self) -> u64 {
        match self {
            Timepoint::BlockNumber(block_number) => *block_number,
            Timepoint::Timestamp(timestamp) => Self::MASK | *timestamp,
        }
    }
}

On-Chain Scripts

• Rollup_state_cell.global_state.tip_block_timestamp equals to l2block.timestamp, same as before.
  • Assert rawl2block.withdrawal.withdrawal_block_number == l2block's timepoint
  • Assert custodian_state_cell.deposit_block_number == l2block's timepoint
  • Assert stake_block_number == l2block's timepoint
• Rollup_state_cell.global_state.last_finalized_block_number
  • is block-number-based, equals to l2block.number - CONSTANT when global_state.version < 2;
  • is timestamp-based, equals to the max timestamp of header_deps, when global_state.version >= 2.
• Determine finality of these entities by comparing their occuration attributions to global_state's last_finalized_block_number if the entity is timestamp-based, otherwise by comparing to global_state's block.count.

[keroro520]

Ready for review @zeroqn @jjyr