Gracefully avoid cases where template parameters are skipped

[adetaylor]

#106 and #411 are cases where bindgen decides that certain template parameters aren't needed.

At the moment, these result in invalid autocxx C++ codegen.

I've realized that we should be able to modify autocxx-bindgen to denote cases where template parameters are skipped, then we can gracefully avoid generating code for those cases. We should still eventually try to fix #106 (really hard) and #411 (impossible).