Renaming Cast to DownCast and adding explicit UpCast expression
Started parser changes
Adding support for type annotations inside comments.

  - Porting Token.hs from parsec to fiddle with `multiLineComment`, so that
    comments starting with "/*:" (or sth equivalent) are not consumed as
    comments.

  - For the moment type annotations are just tugged in the VarDeclStmt, but this
    is easily extensible.
Restored types in parser.