Modules as ADTs

Each module could have distinguished type, that could be accessed by providing only the module name (the same feature as `this` types in Helium). The motivation is to write `List` instead of `List.T` or `List.List` for library-defined types. This feature will work well with #44, as regular ADT could be a module with constructors and single distinguished type. On the other hand, without #34 it would give terrible error messages. The concrete syntax should be determined.