When collecting elements of a stream and producing a summary result (such as a
multi-map with `groupingBy`) it can tricky to map an element downstream to zero
or more elements after which those mapped elements are further operated on.

A `flatMapping` collector that maps an element to a Stream of zero or more
mapped elements can be added to `Collectors`:

    public static <T, U, A, R>
    Collector<T, ?, R> flatMapping(Function<? super T, ? extends Stream<? extends U>> mapper,
                                   Collector<? super U, A, R> downstream) {
        BiConsumer<A, ? super U> downstreamAccumulator = downstream.accumulator();
        return Collector.of(downstream.supplier(),
                            (r, t) -> mapper.apply(t).sequential().forEach(u -> downstreamAccumulator.accept(r, u)),
                            downstream.combiner(),
                            downstream.finisher(),
                            downstream.characteristics().stream().toArray(Collector.Characteristics[]::new));
    }