Fixed warnings

src/parser/ast.rs

@@ -7,7 +7,7 @@ use crate::types::HeapCellValueTag;
 use std::cell::{Cell, Ref, RefCell, RefMut};
 use std::fmt;
 use std::hash::{Hash, Hasher};
-use std::io::{Error as IOError};
+use std::io::Error as IOError;
 use std::ops::{Deref, Neg};
 use std::rc::Rc;
 use std::vec::Vec;