Less memory allocations at runtime

tests/forth/errors.rs

@@ -46,7 +46,7 @@ fn float_literal() {
 fn string_with_spaces() {
     let f = run(r#""hello world" !x @x"#);
     match stack_top(&f) {
-        cagire::forth::Value::Str(s, _) => assert_eq!(s, "hello world"),
+        cagire::forth::Value::Str(s, _) => assert_eq!(s.as_ref(), "hello world"),
         other => panic!("expected string, got {:?}", other),
     }
 }