methods ACCEPTS in class Parcel

It is a bit incomplete since the spec doesn't really say what it should
do; this patch just implements the case where the invocant has no items, which
is also known as Nil.

src/core/Parcel.pm

@@ -2,4 +2,13 @@ augment class Parcel {
     method elems() { self.Seq.elems }
 
     method list() { self.iterator }
+
+    multi method ACCEPTS($x) {
+        # smart-matching against Nil
+        if self.elems == 0 {
+            $x.notdef || ($x.does(::Positional) && $x == 0)
+        } else {
+            die "Don't know how to smart-match against a Parcel that doesn't happen to be empty";
+        }
+    }
 }