pub fn consolidate<T: Ord, R: Semigroup>(vec: &mut Vec<(T, R)>) {
    consolidate_from(vec, 0);
}
Sorts and consolidates `vec`.

This method will sort `vec` and then consolidate runs of more than one entry with
identical first elements by accumulating the second elements of the pairs. Should the final
accumulation be zero, the element is discarded.
differential-dataflow/src/consolidation.rs