Node structure improvements

[rkuris]

Tasks

• BranchNode currently has children and children_hashes. It's impossible for a BranchNode to have a hash of a child but no child address, so perhaps this should be: children: [Option<(LinearAddress, Option<TrieHash>>; Self::MAX_CHILDREN] or something like that
• Investigating removing the Box around BranchNode. This saves some memory but at the cost of some memory allocations during many code paths.
• See if we can improve consuming LeafNode when converting it to a BranchNode
• Investigate any other areas that we can avoid allocating memory