remove `ast` package from `sangria-core`

[performantdata]

After moving most of the ast package's classes into a new module, there were still a few left that didn't belong in that module: AstVisitor, AstVisitorCommand et al. To avoid confusion, these should be moved into a different package. I wasn't sure what package that should be, so I left them in place.

This would best be done after the parser package is put into its own module.