Remove Rc from Parser's vars

This will help enable Parser to be Send, which will be important for
concurrent execution.

src/reader.rs

@@ -5902,7 +5902,7 @@ fn try_expand_wildcard(
     const TAB_COMPLETE_WILDCARD_MAX_EXPANSION: usize = 256;
 
     let ctx = OperationContext::background_with_cancel_checker(
-        &*parser.variables,
+        &parser.variables,
         Box::new(|| signal_check_cancel() != 0),
         TAB_COMPLETE_WILDCARD_MAX_EXPANSION,
     );