Autofixture: Proposal for GenerationDepthBehavior.

Here is a generation Depth limiter behavior proposal.

fixture.Behaviors.Add(new GenerationDepthBehavior(2));

Had to re-implement ComposeIfMultiple for test because it's internal.

```c#
public class GenerationDepthBehavior : ISpecimenBuilderTransformation
{
private const int DefaultGenerationDepth = 1;
private readonly int generationDepth;

public GenerationDepthBehavior() : this(DefaultGenerationDepth)
{
}

public GenerationDepthBehavior(int generationDepth)
{
    if (generationDepth < 1)
        throw new ArgumentOutOfRangeException(nameof(generationDepth), "Generation depth must be greater than 0.");

    this.generationDepth = generationDepth;
}


public ISpecimenBuilderNode Transform(ISpecimenBuilder builder)
{
    if (builder == null) throw new ArgumentNullException(nameof(builder));

    return new GenerationDepthGuard(builder, new GenerationDepthHandler(), this.generationDepth);
}

}

public interface IGenerationDepthHandler
{

object HandleGenerationDepthLimitRequest(object request, IEnumerable<object> recordedRequests, int depth);

}

public class DepthSeededRequest : SeededRequest
{
public int Depth { get; }
public DepthSeededRequest(object request, object seed, int depth) : base(request, seed)
{
Depth = depth;
}
}

public class GenerationDepthGuard : ISpecimenBuilderNode
{
private readonly ThreadLocal> requestsByThread
= new ThreadLocal>(() => new Stack());

private Stack<DepthSeededRequest> GetMonitoredRequestsForCurrentThread() => this.requestsByThread.Value;


public GenerationDepthGuard(ISpecimenBuilder builder)
    : this(builder, EqualityComparer<object>.Default)
{
}

public GenerationDepthGuard(
    ISpecimenBuilder builder,
    IGenerationDepthHandler depthHandler)
    : this(
        builder,
        depthHandler,
        EqualityComparer<object>.Default,
        1)
{
}

public GenerationDepthGuard(
    ISpecimenBuilder builder,
    IGenerationDepthHandler depthHandler,
    int generationDepth)
    : this(
        builder,
        depthHandler,
        EqualityComparer<object>.Default,
        generationDepth)
{
}


public GenerationDepthGuard(ISpecimenBuilder builder, IEqualityComparer comparer)
{
    this.Builder = builder ?? throw new ArgumentNullException(nameof(builder));
    this.Comparer = comparer ?? throw new ArgumentNullException(nameof(comparer));
    this.GenerationDepth = 1;
}


public GenerationDepthGuard(
    ISpecimenBuilder builder,
    IGenerationDepthHandler depthHandler,
    IEqualityComparer comparer)
    : this(
    builder,
    depthHandler,
    comparer,
    1)
{
}

public GenerationDepthGuard(
    ISpecimenBuilder builder,
    IGenerationDepthHandler depthHandler,
    IEqualityComparer comparer,
    int generationDepth)
{
    if (builder == null) throw new ArgumentNullException(nameof(builder));
    if (depthHandler == null) throw new ArgumentNullException(nameof(depthHandler));
    if (comparer == null) throw new ArgumentNullException(nameof(comparer));
    if (generationDepth < 1)
        throw new ArgumentOutOfRangeException(nameof(generationDepth), "Generation depth must be greater than 0.");

    this.Builder = builder;
    this.GenerationDepthHandler = depthHandler;
    this.Comparer = comparer;
    this.GenerationDepth = generationDepth;
}


public ISpecimenBuilder Builder { get; }

public IGenerationDepthHandler GenerationDepthHandler { get; }

public int GenerationDepth { get; }

public int CurrentDepth { get; }

public IEqualityComparer Comparer { get; }

protected IEnumerable RecordedRequests => this.GetMonitoredRequestsForCurrentThread();

public virtual object HandleGenerationDepthLimitRequest(object request, int currentDepth)
{
    return this.GenerationDepthHandler.HandleGenerationDepthLimitRequest(
        request,
        this.GetMonitoredRequestsForCurrentThread(), currentDepth);
}

public object Create(object request, ISpecimenContext context)
{
    if (request is SeededRequest)
    {
        int currentDepth = -1;

        var requestsForCurrentThread = this.GetMonitoredRequestsForCurrentThread();

        if (requestsForCurrentThread.Count > 0)
        {
            currentDepth = requestsForCurrentThread.Max(x => x.Depth) + 1;
        }

        DepthSeededRequest depthRequest = new DepthSeededRequest(((SeededRequest)request).Request, ((SeededRequest)request).Seed, currentDepth);

        if (depthRequest.Depth >= this.GenerationDepth)
        {
            return HandleGenerationDepthLimitRequest(request, depthRequest.Depth);
        }

        requestsForCurrentThread.Push(depthRequest);
        try
        {
            return this.Builder.Create(request, context);
        }
        finally
        {
            requestsForCurrentThread.Pop();
        }
    }
    else
    {
        return this.Builder.Create(request, context);
    }
}

public virtual ISpecimenBuilderNode Compose(
    IEnumerable<ISpecimenBuilder> builders)
{
    var composedBuilder = ComposeIfMultiple(
        builders);
    return new GenerationDepthGuard(
        composedBuilder,
        this.GenerationDepthHandler,
        this.Comparer,
        this.GenerationDepth);
}

internal static ISpecimenBuilder ComposeIfMultiple(IEnumerable<ISpecimenBuilder> builders)
{
    ISpecimenBuilder singleItem = null;
    List<ISpecimenBuilder> multipleItems = null;
    bool hasItems = false;

    using (var enumerator = builders.GetEnumerator())
    {
        if (enumerator.MoveNext())
        {
            singleItem = enumerator.Current;
            hasItems = true;

            while (enumerator.MoveNext())
            {
                if (multipleItems == null)
                {
                    multipleItems = new List<ISpecimenBuilder> { singleItem };
                }

                multipleItems.Add(enumerator.Current);
            }
        }
    }

    if (!hasItems)
    {
        return new CompositeSpecimenBuilder();
    }

    if (multipleItems == null)
    {
        return singleItem;
    }

    return new CompositeSpecimenBuilder(multipleItems);
}

public virtual IEnumerator<ISpecimenBuilder> GetEnumerator()
{
    yield return this.Builder;
}

IEnumerator IEnumerable.GetEnumerator()
{
    return this.GetEnumerator();
}

}

public class GenerationDepthHandler : IGenerationDepthHandler
{
public object HandleGenerationDepthLimitRequest(
object request,
IEnumerable