Function fz_tree_insert 

pub unsafe extern "C" fn fz_tree_insert(
    ctx: *mut fz_context,
    root: *mut fz_tree,
    key: *const c_char,
    value: *mut c_void,
) -> *mut fz_tree

Insert a new key/value pair and rebalance the tree.
Return the new root of the tree after inserting and rebalancing.
May be called with a NULL root to create a new tree.

No data is copied into the tree structure; key and value are
merely kept as pointers.