Refactor out PreimageOracle Interface

packages/contracts-bedrock/contracts/cannon/MIPS.sol

 // SPDX-License-Identifier: MIT
 pragma solidity 0.7.6;
 
-interface IPreimageOracle {
-  function readPreimage(bytes32 key, uint256 offset) external view returns (bytes32 dat, uint256 datLen);
-}
+import { IPreimageOracle } from "./interfaces/IPreimageOracle.sol";
 
 // https://inst.eecs.berkeley.edu/~cs61c/resources/MIPS_Green_Sheet.pdf
 // https://uweb.engr.arizona.edu/~ece369/Resources/spim/MIPSReference.pdf

packages/contracts-bedrock/contracts/cannon/interfaces/IPreimageOracle.sol

+// SPDX-License-Identifier: MIT
+pragma solidity 0.7.6;
+
+/// @title IPreimageOracle
+/// @notice Interface for a preimage oracle.
+interface IPreimageOracle {
+    /// @notice Reads a preimage from the oracle.
+    /// @param _key The key of the preimage to read.
+    /// @param _offset The offset of the preimage to read.
+    /// @return dat_ The preimage data.
+    /// @return datLen_ The length of the preimage data.
+    function readPreimage(bytes32 _key, uint256 _offset)
+        external
+        view
+        returns (bytes32 dat_, uint256 datLen_);
+
+    /// @notice Computes and returns the key for a pre-image.
+    /// @param _preimage The pre-image.
+    /// @return key_ The pre-image key.
+    function computePreimageKey(bytes calldata _preimage) external pure returns (bytes32 key_);
+
+    /// @notice Prepares a preimage to be read by keccak256 key, starting at
+    ///         the given offset and up to 32 bytes (clipped at preimage length, if out of data).
+    /// @param _partOffset The offset of the preimage to read.
+    /// @param _preimage The preimage data.
+    function loadKeccak256PreimagePart(uint256 _partOffset, bytes calldata _preimage) external;
+}