`Fields` (formerly `FieldTypeTuple`) now returns an empty tuple for interfaces

Previously `Fields` returned `AliasSeq!(Interface)` for interfaces as done for
non-aggregate types like `int`, `char*`, ... . This behaviour was surprising
because an instance of an interface *may* have members that just are not
known at compile time.

`Fields` will now return an empty `AliasSeq!()` for interfaces.