Function pack_tree 

pub fn pack_tree<I>(
    pool: &BufferPool,
    tree_id: u16,
    sorted: I,
    fill: f32,
    scratch_dir: &Path,
) -> Result<u32>
where
    I: Iterator<Item = Result<(Vec<u8>, Vec<u8>, bool)>>,

Pack a complete sorted tree. Kept as the stable whole-tree primitive used by bulk load and recovery; range grafting extends it through pack_range instead of duplicating its page builder.