Namespaces: final work to hierarchize cxxbridge internal mod

[adetaylor]

After #76 we will still have a bit more namespace work to do. The final output structure should be correct, but within our internal cxxbridge mod we'll still have a flat structure. That means we won't allow input where there are identically-named symbols in different namespaces.

To finally fix that, we need to do the mod parts of https://github.com/dtolnay/cxx/issues/353 and then reflect in autocxx.