Make Amp\Struct abstract to better represent its purpose

lib/Struct.php

@@ -2,7 +2,14 @@
 
 namespace Amp;
 
-class Struct {
+/**
+ * A "safe" Struct class for public property aggregators
+ * 
+ * This class is intended to make using public properties a little safer by throwing when
+ * nonexistent property names are read or written. All property aggregation classes in the
+ * Amp library descend from Struct somewhere in their inheritance heirarchies.
+ */
+abstract class Struct {
     final public function __get($property) {
         throw new \DomainException(
             $this->generatePropertyError($property)