prune pending chain if needed during propose and prove

[just-mitch]

Add L1 checks if the pending chain should be pruned when a new pending block is proposed, or a proof of epoch is submitted.

For this, we will need a skeleton Escrow contract with the following interface:

interface IEscrow {
    function deposit(uint256 amount, uint256 deadline) external;
    function stakeBond(Quote calldata quote) external returns (address); // returns the bond provider
    function unstakeBond(address bondProvider, uint256 amount) external;
    function withdraw(uint256 amount) external;
}

For the immediate term, these can largely be no-ops.

The purpose of this issue is to ensure that it is impossible to submit a pending block or a proof when there is a pending chain that can be pruned.