Support for private/internal constructors

At least support for singletons
```C#
public sealed class RecoveryCompleted
{
public static RecoveryCompleted Instance { get; } = new RecoveryCompleted();
private RecoveryCompleted() {}
}
```
https://github.com/neuecc/MessagePack-CSharp/issues/40

Review the serializer's handling of constructors and the linked MessagePack-CSharp issue for the expected behavior. Confirm that the singleton example with a private constructor can be serialized and deserialized without requiring a public constructor.

Written by the indexing model from the issue text.