### A validator for a Ludus AST

(comment

Tracking here, before I start writing this code, the kinds of validation we're hoping to accomplish:

* splats come at the end of list, tuple, and dict patterns
* no unbound names
* no re-bound names
* correct imports
* no unbound names with `use` forms
* no circular imports
* first-level property access with pkg, e.g. `Foo :bar`--bar must be on Foo

)