Use pool of ObjectTraverseStacks in ToObject().

[StringEpsilon]

For some workloads, ObjectInstance.ToObject() causes the Stack<object> internal to ObjectTraverseStack to allocate additional memory, which is then promptly disposed of, only to be re-allocated again on the next call that requires the conversion.

Using a pool reduces the amount of allocation and, for the workload I am optimizing for, makes ToObject() so fast that it vanishes from the profiler.

[StringEpsilon]

Benchmark:

[MemoryDiagnoser]
public class ToObjectBench
{
    private Engine _engine = new Engine().SetValue("test", new Dictionary<int, object>());

    [Benchmark]
    public void InteropCall() =>
        _engine.Execute("""
            for(var i = 0; i < 10; i++) {
                test.Add(i, {});
            }
            test.Clear();
            """);
}

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
main	6.500 us	0.0278 us	0.0247 us	1.3657	0.0229	22.35 KB
branch	6.142 us	0.0468 us	0.0415 us	1.0757	0.0153	17.66 KB

I also ran the interop benchmark set, but they showed no difference in runtime with only one test showing a very small decrease in allocated memory.

[lahma]

I think ObjectPool is not thread-safe so it shouldn't be exposed as static field (static via ownership), Engine field would be safe. Needs rebase.

[lahma]

Thank you!