merge feature/tree_add branch

rework cx_tree_add() API to allow insertion of edge nodes feature/tree_add

remove condition that is always false after recent fix feature/tree_add

fix cx_tree_search() not remembering root as a valid candidate feature/tree_add

cx_tree_add_iter() - optimize check for empty trees feature/tree_add

cx_tree_add() fix missing spec for adding duplicates feature/tree_add

complete cx_tree_add() implementations feature/tree_add

fix return type of cx_tree_node_create_func feature/tree_add

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