optimize ListTreeNode Children for large JSON

[ousttrue]

No description provided.

[yutopp]

How can I check that the logic fixed by this PR is optimized?

[ousttrue]

Collection(Array/Object) that has many member's member access is slow.
Nested ...

var f = new JsonFormatter();
f.BeginObject();
f.Key("values");
f.BeginArray();
for(int i=0; i<10000; ++i)
{
    f.Value(i);
}
f.EndArray();
f.EndObject();
var parsed = f.GetStoreBytes().ParseAsJSon();

parsed["values"].ArrrayItem().Count() // <- this is slow
}

because

                int count = 0;

                //
                // before: All value enumerated
                // for (int i = ValueIndex; i < m_Values.Count; ++i) 
                //
                for (int i = ValueIndex; count < ChildCount && i < m_Values.Count; ++i)
                {	                {
                    if (m_Values[i].ParentIndex == ValueIndex)	                    if (m_Values[i].ParentIndex == ValueIndex)
                    {	                    {
                        ++count;
                        yield return new ListTreeNode<T>(m_Values, i);	                        yield return new ListTreeNode<T>(m_Values, i);
                    }	                    }
                }	                }

1ca7c20#diff-d7dfb11fb75a11ebf30df6d885eedab3R346