Import and export improved

- Import paths are now vname and not sym
- Imports and exports accept multiple space-delimited operators in []

as a result, we can now reliably import and export the operator *

- error reporting ergonomics

src/error/unexpected_directory.rs

@@ -1,7 +1,7 @@
-use super::{ErrorPosition, ProjectError};
-use crate::utils::iter::box_once;
-use crate::utils::BoxedIter;
-use crate::{Interner, VName};
+use std::rc::Rc;
+
+use super::ProjectError;
+use crate::{Interner, Location, VName};
 
 /// Produced when a stage that deals specifically with code encounters
 /// a path that refers to a directory
@@ -15,9 +15,10 @@ impl ProjectError for UnexpectedDirectory {
     "A stage that deals specifically with code encountered a path that refers \
      to a directory"
   }
-  fn positions(&self, i: &Interner) -> BoxedIter<ErrorPosition> {
-    box_once(ErrorPosition::just_file(i.extern_all(&self.path)))
+  fn one_position(&self, i: &Interner) -> crate::Location {
+    Location::File(Rc::new(i.extern_all(&self.path)))
   }
+
   fn message(&self, i: &Interner) -> String {
     format!(
       "{} was expected to be a file but a directory was found",