Template declaration of std.typecons.isTuple in -X JSON file.

[realhet]

```d
struct Tuple(T...){ T expand;alias expand this; }

pragma(msg, isTuple!(Tuple!(int)));
```

And LDC2's opinion about Your Tuple is: Not a tuple :D

These are the mysterious parts of the language, I like this 
stuff, even I can't understand.