This class enables printing the import list in generated files
in a smarter way.
Also, refactor `Importable` interface hierarchy, namely, don't inherit
`TypeRef` from `Importable`, since we have types like `TypeRef.Star`
which are not really importable.
Replace the `Importable#typeWithArguments` property with
the `TypeRef#render` method to utilize `ImportCollector` while rendering
types.