add a toString() override

this was needed for compatability with [email protected] and up,
where Object.create() is used when creating ref types and deref types

lib/struct.js

@@ -98,7 +98,9 @@ function Struct () {
   constructor.prototype.constructor = constructor
 
   constructor.defineProperty = defineProperty
+  constructor.toString = toString
   constructor.fields = {}
+
   // Setup the ref "type" interface. The constructor doubles as the "type" object
   constructor.size = 0
   constructor.alignment = 0
@@ -164,6 +166,10 @@ function set (buffer, offset, value) {
   }
 }
 
+function toString () {
+  return 'StructType'
+}
+
 /**
  * Adds a new field to the struct instance with the given name and type.
  * Note that this function will throw an Error if any instances of the struct