Cleanup pass based on prior PRs (#21)

* Updates to stub generation

* More idiomatic fixes from PHP team

* Moved tests from src/tests -> tests, got rid of extension-only tests

* fixed up test paths, regenerated arginfo

src/AWS/CRT/HTTP/Headers.php

@@ -1,5 +1,8 @@
 <?php
-
+/**
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0.
+ */
 namespace AWS\CRT\HTTP;
 
 use AWS\CRT\Internal\Encoding;