Transiton into Typescript's external modules

[kskalski]

Currently generated code and mscorlib assume they will be used as "internal modules", which is nowadays discouraged / hard to use in new Typescript code:

• reference to mscorlib is done through ///
• generated classes are not exported

Such modules seems to be impossible to consume using "import" statements. It would be nice to have an command line switch to make the generated code export its classes. Ideally mscorlib should albo be exporting its classes (and then generated code would consume them by importing them first), but that would break compatibility with old generated code, which uses ///<reference.. to mscorlib.