Implement a custom SortedList enumerator to reduce allocations

[smoogipoo]

             Method |     Mean |     Error |    StdDev |    Gen 0 | Allocated |
------------------- |---------:|----------:|----------:|---------:|----------:|
 NoCustomEnumerator | 67.27 ms | 0.3430 ms | 0.3209 ms | 125.0000 |  400000 B |
   CustomEnumerator | 11.46 ms | 0.0927 ms | 0.0774 ms |        - |       0 B |

    [MemoryDiagnoser]
    public class Bench
    {
        private SortedList<int> sortedList = new SortedList<int>();
        private SortedListCustomEnumerator<int> sortedListCustomEnumerator = new SortedListCustomEnumerator<int>();

        [GlobalSetup]
        public void Setup()
        {
            for (int i = 0; i < 1000; i++)
            {
                sortedList.Add(i);
                sortedListCustomEnumerator.Add(i);
            }
        }

        [Benchmark]
        public void NoCustomEnumerator()
        {
            int finalValue = 0;

            for (int i = 0; i < 10000; i++)
            {
                foreach (var val in sortedList)
                    finalValue += val;
            }
        }

        [Benchmark]
        public void CustomEnumerator()
        {
            int finalValue = 0;

            for (int i = 0; i < 10000; i++)
            {
                foreach (var val in sortedListCustomEnumerator)
                    finalValue += val;
            }
        }
    }

Providing a custom enumerator makes the compiler not box. (https://stackoverflow.com/a/13752402)

Note that this is bypassed if one of the interfaces of SortedList<T> is ever used. E.g. this is bypassed by autosize, which references IReadOnlyList<T> AliveInternalChildren rather than SortedList<T> aliveInternalChildren. But this happens for List<T> too.