Extern type defined with generic parameters incorrectly generated in C++

[rdegnan]

This commit fixes #149

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[artwyman]

Hi @rdegnan. Thanks for taking a stab at this. Looks like this change breaks some existing functionality, though. When I try to build the sample app I see many errors like this:

/Users/atwyman/src/djinni/example/generated-src/cpp/item_list.hpp:13:10: error: use of class template 'std::vector'
      requires template arguments
    std::vector items;

Looks like your change is affecting the handling of template args for types like vector<> in a bad way.

For future reference, when making any changes, please make sure you can make all from the top-level of the repo (which can only run on a Mac due to the inclusion of iOS stuff). That'll re-generate and build the example apps and test suite, and run the tests. Note that you should also check in the resulting generated code. That's not a normal best-practice for generated code, but in this case, reviewing the generated code for the example and test suite is the easiest way for a reviewer like me to see the net effect of any changes to the generator.

[artwyman]

See my prior comment for details. Please re-submit after resolving build/test problems, and also include generated code changes for example app and test suite.