PERF: StackyRecursive.cs

[perf-cat]

Hi @askazakov, I've tried to make some performance optimizations to this repo. Following are the benchmark results before and after the changes:

Benchmarks before changes:

Method	Mean	Error	StdDev	Median	Gen 0	Gen 1	Gen 2	Allocated
StackyRecursiveBenchmark	199.7 ns	4.03 ns	8.42 ns	196.5 ns	0.1566	-	-	328 B

Benchmarks after changes:

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
StackyRecursiveBenchmark	115.7 ns	2.27 ns	2.33 ns	0.0994	-	-	208 B

As you can see the allocations have gone down. I've also made sure that all the unit tests pass following these changes as well. Could you please confirm whether or not you think my changes are valid?

Thank you!