token: ditch the generic bundle in favor of specialized struct

It's kust cleaner that way, the generic has no upside.

token/delegation/utilities.go

@@ -15,3 +15,10 @@ type Loader interface {
 	// If not found, ErrDelegationNotFound is returned.
 	GetDelegation(cid cid.Cid) (*Token, error)
 }
+
+// Bundle carries together a decoded token with its Cid and raw signed data.
+type Bundle struct {
+	Cid     cid.Cid
+	Decoded *Token
+	Sealed  []byte
+}