Less memory allocations at runtime

tests/forth/definitions.rs

@@ -128,7 +128,7 @@ fn forget_removes_word() {
     let stack = f.stack();
     assert_eq!(stack.len(), 1);
     match &stack[0] {
-        Value::Str(s, _) => assert_eq!(s, "double"),
+        Value::Str(s, _) => assert_eq!(s.as_ref(), "double"),
         other => panic!("expected Str, got {:?}", other),
     }
 }