Vector Downward GetSize optimization (#6925)

* Added Google benchmarks (and gtests)

* Separate benchmark CMakeLists.txt to its own file

* Move output directory to target just flatbenchmark

* Reduced from encoding 210ns -> 188ns

* store size_ as uoffset_t

* fixed windows c4267 warning

benchmarks/cpp/benchmark_main.cpp

@@ -93,4 +93,4 @@ static void BM_Raw_Use(benchmark::State &state) {
   std::unique_ptr<Bench> bench = NewRawBench();
   Use(state, bench, buffer, 218812692406581874);
 }
-BENCHMARK(BM_Raw_Use);
+BENCHMARK(BM_Raw_Use);