support strings as char lists in term comparisons

src/prolog/string_list.rs

@@ -24,7 +24,8 @@ pub struct StringList {
 
 impl Hash for StringList {
     fn hash<H: Hasher>(&self, state: &mut H) {
-        self.borrow().hash(state)
+        let h = self.borrow().hash(state);
+        (h, self.cursor, self.expandable).hash(state);
     }
 }
 
@@ -63,11 +64,6 @@ impl StringList {
         }
     }
 
-    #[inline]
-    pub fn cursor(&self) -> usize {
-        self.cursor
-    }
-
     #[inline]
     pub fn head(&self) -> Option<char> {
         self.borrow()[self.cursor ..].chars().next()