style(ctb): clean lint in MerkleTrie (#3028)

Cleans up minor linting issues in MerkleTrie.sol.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/dirty-bananas-shop.md

+---
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Cleans linting errors in MerkleTrie.sol

packages/contracts-bedrock/contracts/libraries/trie/MerkleTrie.sol

@@ -23,42 +23,42 @@ library MerkleTrie {
     /**
      * @notice Determines the number of elements per branch node.
      */
-    uint256 constant TREE_RADIX = 16;
+    uint256 internal constant TREE_RADIX = 16;
 
     /**
      * @notice Branch nodes have TREE_RADIX elements and one value element.
      */
-    uint256 constant BRANCH_NODE_LENGTH = TREE_RADIX + 1;
+    uint256 internal constant BRANCH_NODE_LENGTH = TREE_RADIX + 1;
 
     /**
      * @notice Leaf nodes and extension nodes have two elements, a `path` and a `value`.
      */
-    uint256 constant LEAF_OR_EXTENSION_NODE_LENGTH = 2;
+    uint256 internal constant LEAF_OR_EXTENSION_NODE_LENGTH = 2;
 
     /**
      * @notice Prefix for even-nibbled extension node paths.
      */
-    uint8 constant PREFIX_EXTENSION_EVEN = 0;
+    uint8 internal constant PREFIX_EXTENSION_EVEN = 0;
 
     /**
      * @notice Prefix for odd-nibbled extension node paths.
      */
-    uint8 constant PREFIX_EXTENSION_ODD = 1;
+    uint8 internal constant PREFIX_EXTENSION_ODD = 1;
 
     /**
      * @notice Prefix for even-nibbled leaf node paths.
      */
-    uint8 constant PREFIX_LEAF_EVEN = 2;
+    uint8 internal constant PREFIX_LEAF_EVEN = 2;
 
     /**
      * @notice Prefix for odd-nibbled leaf node paths.
      */
-    uint8 constant PREFIX_LEAF_ODD = 3;
+    uint8 internal constant PREFIX_LEAF_ODD = 3;
 
     /**
      * @notice RLP representation of `NULL`.
      */
-    bytes1 constant RLP_NULL = bytes1(0x80);
+    bytes1 internal constant RLP_NULL = bytes1(0x80);
 
     /**
      * @notice Verifies a proof that a given key/value pair is present in the trie.
@@ -125,6 +125,7 @@ library MerkleTrie {
      * @return Portion of the key remaining after the walk.
      * @return Whether or not we've hit a dead end.
      */
+    // solhint-disable-next-line code-complexity
     function _walkNodePath(
         TrieNode[] memory _proof,
         bytes memory _key,