ADTs as modules

Each definition of a ADT could also be a definition of a module that contains all the constructors. It is possible due to fact that types and modules form separate namespaces that both start with an uppercase letter. The motivation is that it would be easier to work with multiple data-types with the same names of constructors.
```
data T = A | B
data U = A | B

let x = T.A
let y = U.B
```