Support RLP-encoded block header validation against `blockHash`

[0xClandestine]

🧐 Motivation

It would be valuable if the Blockhash utility supported verifying RLP-encoded block headers against historical block hashes. This is useful for block header, account, storage, tx, and event proofs.

📝 Details

Proposed implementation (should definitely include calldata variants given rlp block headers are typically provided by the user as calldata):

/**
 * @dev Verifies that the RLP-encoded block header matches the canonical block hash.
 * @param blockHeader The full RLP-encoded block header.
 * @param blockNumber The block number it claims to correspond to.
 * @return True if the keccak256 hash of the blockHeader matches the historical block hash.
 */
function verifyBlockHeader(bytes memory blockHeader, uint256 blockNumber) internal view returns (bool) {
    return blockHeader.blockHash() == blockNumber.blockHash();
}

/**
 * @dev Computes the hash of an RLP-encoded block header.
 * @param rlp The RLP-encoded header data.
 * @return hash The keccak256 hash of the RLP payload.
 */
function blockHash(bytes memory rlp) internal pure returns (bytes32 hash) {
    assembly {
        hash := keccak256(add(rlp, 0x20), mload(rlp))
    }
}

This addition would complement #5680.

[0xClandestine]

Probably a good idea to include something like this as well. Was able to significantly reduce the cost to decode the most commonly used header fields (e.g. stateRoot, txRoot, recieptsRoot, and logsBloom) by hardcoding their offsets/lengths.

/// @notice Efficiently extracts one of the first 7 fields from an RLP-encoded Ethereum block header.
/// @dev Optimized for extracting one of the first 7 fields (parentHash, ommersHash, etc.)
///      from a canonical RLP-encoded block header using precomputed offsets.
///      Much faster and cheaper than generic RLP decoding for these fields.
function decodeLeadingField(bytes calldata header, uint256 field) internal pure returns (bytes memory) {
    unchecked {
        // Field 0: parentHash (32 bytes)
        // Field 1: ommersHash (32 bytes)
        // Field 2: beneficiary (20 bytes)
        // Field 3: stateRoot (32 bytes)
        // Field 4: transactionsRoot (32 bytes)
        // Field 5: receiptsRoot (32 bytes)
        // Field 6: logsBloom (259 bytes)
        uint8[7] memory offsets = [1, 34, 67, 88, 121, 154, 186];
        uint16[7] memory lengths = [32, 32, 20, 32, 32, 32, 259];
        uint256 start = 1 + (uint8(header[0]) - (LIST_LONG_START - 1)) + offsets[field];
        return header[start:start + lengths[field]];
    }
}

[0xClandestine]

cc @ernestognw