struct tupleof constructor

[p0nce]

Adam uses:

struct S
{
    this(typeof(this.tupleof) args)
    {
        this.tupleof = args;
    }
    // members...
}
```
So that using the struct construction syntax catch errors with args.

See beginning of https://www.youtube.com/watch?v=XuYcZPYWbKk