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/visibility_mismatch.rs

@@ -1,9 +1,7 @@
 use std::rc::Rc;
 
-use super::project_error::{ErrorPosition, ProjectError};
+use super::project_error::ProjectError;
 use crate::representations::location::Location;
-use crate::utils::iter::box_once;
-use crate::utils::BoxedIter;
 use crate::{Interner, VName};
 
 /// Multiple occurences of the same namespace with different visibility
@@ -18,13 +16,13 @@ impl ProjectError for VisibilityMismatch {
   fn description(&self) -> &str {
     "Some occurences of a namespace are exported but others are not"
   }
-  fn positions(&self, i: &Interner) -> BoxedIter<ErrorPosition> {
-    box_once(ErrorPosition {
-      location: Location::File(self.file.clone()),
-      message: Some(format!(
-        "{} is opened multiple times with different visibilities",
-        i.extern_all(&self.namespace).join("::")
-      )),
-    })
+  fn message(&self, i: &Interner) -> String {
+    format!(
+      "{} is opened multiple times with different visibilities",
+      i.extern_all(&self.namespace).join("::")
+    )
+  }
+  fn one_position(&self, _i: &Interner) -> Location {
+    Location::File(self.file.clone())
   }
 }