Add a faster implementation of the union-find for Serial

[aprokop]

The patch tries to solve the problem of slow Serial atomics when Kokkos compiled with other backends. This patch is not strictly necessary right now, as one could compile Kokkos with just the Serial backend. However, it becomes necessary once I introduce a dendrogram construction algorithm using union-find, which will run MST construction on the device, but construct union-find on CPU using a single thread. In that context, it's not possible to fix it by just compiling Kokkos.

Drive-by change: labels_ -> _labels

[aprokop]

The following code for 20,000,000 edges went from 0.46 to 0.23 when using Serial backend but compiled with others.

    Kokkos::parallel_for(
        "Bechmark::union-find",
        Kokkos::RangePolicy<ExecutionSpace>(ExecutionSpace{}, 0, num_edges),
        KOKKOS_LAMBDA(int e) {
          int i = edges(e).source;
          int j = edges(e).target;

          for (int k : {i, j})
          {
            auto child = labels(union_find.representative(k));
            if (child != UNDEFINED)
              parents(child) = e;
          }

          union_find.merge(i, j);
          labels(union_find.representative(i)) = e;
        });