Function bag_peaks 

pub fn bag_peaks(hasher: &dyn Hasher, peaks: &[Vec<u8>], k: u64) -> Vec<u8> 

Bag a frontier’s peaks into the single member root under hasher.

Evaluates bag_shape with each nary_mr node hashing its children. A single peak promotes to itself; an empty peak set is the empty-tree root (hasher.empty()).