We don't allow a lambda variable to be polymorphic anymore. Of course
this isn't bad, but it makes things too difficult, at least for now.
It's also likely that we don't need this specific feature very often I
think.
This ports TestAstFunc2 from our home-grown content storage system to
the txtar package. Since a single file can be used to represent the
entire folder hierarchy, this makes it much easier to see and edit
tests.