Go generating backend

[crawshaw]

Now that plugin.Open works for importing Go packages, we can implement importing Neugram packages with a Go generating backend.

That is, when we see import 'mypkg.ng', create a Go package from mypkg.ng, build it, and then use plugin.Open.

This works around the fact that Go's reflect package cannot yet create methods, so we get good methodik support in imported packages.