Namespaces

[adetaylor]

A big intentional omission from the autocxx work so far has been C++ namespaces.

I'm now tackling this.

[adetaylor]

Working on this here - dtolnay/cxx#370 - which will require support from autocxx too afterwards.

[adetaylor]

Currently working on this. Not much to show yet.

[adetaylor]

This will be closed by #70, but there's still more work to do which will be tracked by #37.