Remove Outbox.checkpoint

[asaj]

• Keep latestCheckpoint(), which should calculate the root and index
• Remove helper functions in Router.sol and call sites (note in a/the changelog for when we publish sdk version)
• Add function for slashing
• remove all the superflous "first message enqueues" on an outbox that we added to avoid the revert for checkpointing a single leaf tree

Docs changes:

• Remove checkpointer agent section
• Remove checkpointing mentions
• Update Diagram

Infra Changes: (probably file another ticket)

• remove checkpointer role
• remove checkpointer agent config
• remove checkpointer deployment config

Rust changes: (probably file another ticket)

• Remove checkpointer binary
• Update ABI and contract wrappers

[asaj]

Started work in asaj/death-to-checkpoints

[asaj]

From @tkporter, suggesting that some notion of checkpoints should still exist to prevent validators from effectively being able to censor slashing txs:

If so, require(_calculatedLegitimateRoot == root()); on line 28 could be concerning, because without checkpoints the calls to the slash function could be frontran by a transaction that adds a message to the Outbox's merkle tree, causing root() to be different from the _calculatedLegitimateRoot from the proof. So the implementation may need some notion of checkpointing for slashing purposes to protect from malicious frontrunning? I.e. prior to slashing, you first store historicalRoot[root()] = true, then in a subsequent transaction you call slash but line 28 is instead require(historicalRoot[_calculatedLegitimateRoot]) (edited)