Non-POD structure constants

[adetaylor]

A follow up from #93, much harder.

We need to figure out how to expose C++ constants which are types that cannot be represented in Rust by value.

We probably need to generate some C++ code which will return a UniquePtr to a copy of the constant. This assumes that the constant can be copied, and obviously has performance implications.

I'm adding an #[ignore]d test - test_non_pod_constant.