Improve performance of ChooseK algorithm

[hatyo]

The algorithm is maintaining its state by using few iterators that are allocated and reallocated too many times, the same state could be maintained by simple using integer counters. This could reduce memory consumption and make the algorithm a bit faster.

I wrote a simple synthetic benchmark to verify this, the benchmark uses chooseK to choose 10 items from a set of 28 (~13 millions combinations), and repeats for ten times. Here is the benchmark:

@Test
void testChooseKStressOld() {
    final var elementsBuilder = ImmutableSet.<String>builder();
    IntStream.range(1, 28 + 1 /* exclusive */).forEach(i -> elementsBuilder.add("e" + i));
    final Set<String> elements = elementsBuilder.build();
    final EnumeratingIterable<String> combinationsIterable = ChooseK.chooseK(elements, 10);
    int cnt = 0;
    do {
        long numCombinations = 0;
        final EnumeratingIterator<String> iterator = combinationsIterable.iterator();
        while (iterator.hasNext()) {
            iterator.next();
            numCombinations++;
        }
        Assertions.assertEquals(13123110L, numCombinations);
    } while (cnt++ < 10);
}

It showed that indeed we reduce the memory consumption, and get almost 15% performance increase, pictures below shows differential view between current algorithm and the refactored one using only integers of both memory and CPU measurements.