Converted Interner to work with Rc-s

- Interner no longer contains unsafe code
- Tokens now hold a reference to the value they represent directly

This will enable many future improvements

src/error/not_exported.rs

@@ -21,21 +21,23 @@ impl ProjectError for NotExported {
   fn description(&self) -> &str {
     "An import refers to a symbol that exists but isn't exported"
   }
-  fn positions(&self, i: &Interner) -> BoxedIter<ErrorPosition> {
+  fn positions(&self) -> BoxedIter<ErrorPosition> {
     Box::new(
       [
         ErrorPosition {
-          location: Location::File(Rc::new(i.extern_all(&self.file))),
+          location: Location::File(Rc::new(Interner::extern_all(&self.file))),
           message: Some(format!(
             "{} isn't exported",
-            i.extern_all(&self.subpath).join("::")
+            Interner::extern_all(&self.subpath).join("::")
           )),
         },
         ErrorPosition {
-          location: Location::File(Rc::new(i.extern_all(&self.referrer_file))),
+          location: Location::File(Rc::new(Interner::extern_all(
+            &self.referrer_file,
+          ))),
           message: Some(format!(
             "{} cannot see this symbol",
-            i.extern_all(&self.referrer_subpath).join("::")
+            Interner::extern_all(&self.referrer_subpath).join("::")
           )),
         },
       ]