HTTP message API (#19)

* HTTP message API

* Added MacOS CI

* Grabbed fixes for mac from aws-crt-ffi

src/AWS/CRT/HTTP/Headers.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace AWS\CRT\HTTP;
+
+use AWS\CRT\Internal\Encoding;
+
+final class Headers {
+    private $headers;
+
+    public function __construct($headers = []) {
+        $this->headers = $headers;
+    }
+
+    public static function marshall($headers) {
+        $buf = "";
+        foreach ($headers->headers as $header => $value) {
+            $buf .= Encoding::encodeString($header);
+            $buf .= Encoding::encodeString($value);
+        }
+        return $buf;
+    }
+
+    public static function unmarshall($buf) {
+        $strings = Encoding::readStrings($buf);
+        $headers = [];
+        for ($idx = 0; $idx < count($strings);) {
+            $headers[$strings[$idx++]] = $strings[$idx++];
+        }
+        return new Headers($headers);
+    }
+
+    public function count() {
+        return count($this->headers);
+    }
+
+    public function get($header) {
+        return isset($this->headers[$header]) ? $this->headers[$header] : null;
+    }
+
+    public function set($header, $value) {
+        $this->headers[$header] = $value;
+    }
+
+    public function toArray() {
+        return array_merge($this->headers);
+    }
+}