Eigen support

[johnwason]

Eigen linear algebra template library is very important for scientific and engineering applications. I suggest "But does it run Eigen?" as a test of interoperability between Carbon and C++ templates because of its importance, and also because it is a good example of a complex template library.

[jonmeow]

Honestly I think Eigen is a popular enough library that we'll run into it while trying to seriously test interop. But it's pretty far off right now that we'll be doing that.