Add BTree::insert_node_as_right_sibling_of().
Add BTree::transfer_leaf_tuples_right().
Add BTree::selectively_insert_tuple().
Tidy up the tree-printing functionality.
Improved BTree::add_root_node().
Rename some variables.
Change InsertResult into Split.
Tidy up a little.
Tidy up and comment the Split struct.