Writing include guards around each type.

[adetaylor]

This avoids problems with duplicate definitions when the
generated .cc ends up including the generated .h via intermediate
includes.

I may yet add another commit here to add some test code.

Fixes #230.

[dtolnay]

Thank you! I'll merge this as is, with just a small tweak I pushed.

On testing -- one thing I would love to get is a representative integration test project. The test crate we have in https://github.com/dtolnay/cxx/tree/0.3.4/tests/ffi is geared toward exhaustively covering the code generator to tell that it produces compilable code for all the combinations of types you could pass/return, by value/by reference, rust->c++ / c++->rust, infallible/fallible, etc. But so far it makes no attempt to imitate a realistic project structure with multiple libraries involved. As things get somewhat figured out on your end, it would be amazing if you could share links or brief explanations to help reproduce something resembling that structure as a test project here.