POD structure constants

[adetaylor]

We should support C++ string constants. We already support int constants.

Later today I will commit an #[ignore]d test for this, integration_tests::test_pod_constant.

bindgen does output the constant, like this:

        extern "C" {
            #[link_name = "\u{1}__ZL3BOB"]
            pub static BOB: root::Bob;
        }

so it should just be a matter of educating bridge_converter.rs that this is OK, and adding a use in the final output ffi mod.