chore: add RevealedSparseTrie::insert_branch_node

[Rjected]

This splits out the branch node insertion logic in reveal_node, into a dedicated function. Also adds some more docs related to reveal_node_or_hash

[rkrasiuk]

@Rjected are we planning to reuse this function anywhere? otherwise, seems like a minor misdirection

[Rjected]

@Rjected are we planning to reuse this function anywhere? otherwise, seems like a minor misdirection

No, maybe skill issue but when reading the code previously it was not immediately clear to me that this is what the match was doing. As well as the branches for the other node types. Open to just adding docs, but I think if the match in reveal_node looked like the following:

        match node {
            TrieNode::EmptyRoot => {
                debug_assert!(path.is_empty());
                self.nodes.insert(path, SparseNode::Empty);
            }
            TrieNode::Branch(branch) => {
                // first few lines
                self.insert_branch_node(branch, path, masks)?;
            }
            TrieNode::Extension(ext) => {
                self.insert_extension_node(ext, path, masks)?;
            }
            TrieNode::Leaf(leaf) => {
                self.insert_leaf_node(leaf, path, masks)?;
            }
        }

it would have been easier to read / understand