Consider whether to switch IsSetInfo to use a class instead of a struct

[adamconnelly]

The compiler generates an IsSetInfo struct containing a member for each optional field in the Thrift struct. At the moment it uses a C# struct rather than a class, to match what the official compiler does. The issue with this is that it breaks the guidelines for when to use a struct in C#.

Here's an example:

public class User
{
    private IsSetInfo isSet;
    private int? _Field1;

    /// <summary>
    /// Gets information about whether each optional field has been set.
    /// </summary>
    public IsSetInfo IsSet
    {
        get { return this.isSet; }
    }

    public int? Field1
    {
        get 
        {
            return _Field1;
        }

        set
        {
            this._Field1 = value;
            this.isSet.Field1 = true;
        }
    }

    /// <summary>
    /// Contains an entry for each optional field, indicating whether it has been
    /// set or not.
    /// </summary>
    public struct IsSetInfo
    {
        public bool Field1;
    }
}

Because of that, we should try to figure out why this decision was made, and if it makes sense to keep it as a struct, we should document the reason why. In case a struct was chosen for performance reasons, we should wait to make any changes until we have benchmarks in place so we can justify the change makes sense.