fix unmarshal of top-level "root" scope

Closes #9

zo-lib/compiler/zo-parse.rkt

@@ -1175,7 +1175,7 @@
        (struct-copy prefix p [stxs (map walk s)])]
       [(req rs _)
        (struct-copy req p
-                    [reqs (map walk rs)])]
+                    [reqs (walk rs)])]
       [(? mod?)
        (struct-copy mod p
                     [prefix (walk (mod-prefix p))]
@@ -1298,6 +1298,9 @@
                  [_ (error 'decode-wrap "bad shift")]))))
 
 (define (decode-scope s ht)
+  (or
+   (and (eq? s root-scope)
+        s)
    (hash-ref ht s
              (lambda ()
                (unless (encoded-scope? s)
@@ -1344,7 +1347,7 @@
                                        (decode-binding (cdr p) ht)))
                                (loop (+ i 2)))])))
                  (set-scope-bindings! sc bindings))
-              sc)))
+               sc))))
 
 (define (decode-scope-set l ht)
   (decode-map decode-scope l ht))