Implemented async callbacks to PHP regardless of thread model, Sigv4 signing (#24)

* Updated to get latest aws-crt-ffi

* Added implementation for signing_result

* Added main thread queue and callback queueing to signing process

* Updated to get support for date on signing_config_aws

* added date support to SigningConfigAWS, fixed up reference issues on http message signing

* Broke C code into separate files, moved common stuff to php_aws_crt.h

* Use a unity build to make PHP do the right thing with our lib

* Updated aws-crt-ffi to v0.1.3 to get sigv4 changes

* Added CRT::isAvailable for safe runtime detection of CRT

* Fixed up test resource safety, added tests to prove it

* Added CI jobs for every version of PHP on linux (#26)

tests/000_CoreTest.php

@@ -0,0 +1,22 @@
+<?php
+/**
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0.
+ */
+use AWS\CRT\CRT;
+
+require_once('common.inc');
+
+// This intentionally does not inherit from CrtTestCase because it needs a clean-room environment
+final class CoreTest extends PHPUnit_Framework_TestCase {
+
+    // The CRT should always be available in this test suite
+    public function testIsAvailable() {
+        $this->assertTrue(CRT::isAvailable());
+    }
+
+    // We have done nothing to necessitate loading the CRT, it should not be loaded
+    public function testIsLoaded() {
+        $this->assertTrue(!CRT::isLoaded());
+    }
+}