Fix Issue 13068 - std.typecons.Unique should disable postblit

https://issues.dlang.org/show_bug.cgi?id=13068

* Fix struct unittest.
* Fix class unittest.
* Disable `Unique` postblit.
You're right, I removed the comments in 172f20e.

I've also reverted the bugs section about not implicitly returning Unique - this does actually work for returning a local Unique, as evidenced by `release`. It only doesn't work when returning a Unique *parameter*. In short, the line I quoted is correct after all.