Rename testcases for (automatic) casting
Move literals/*-testcases to the right folders
Fix typecheck on explicitly upcasting

```instance as SomeClass```
The check in TypeCheckVisitor would only check if `SomeClass` inherits the
class of `instance` but not the other way arround.
Add testcases for explicit upcasting
Merge pull request #2 from lummax/fix_upcast_with_as_operator

Fix upcast with 'as' operator

Merged after 30 days.