HTTP message API (#19)

* HTTP message API

* Added MacOS CI

* Grabbed fixes for mac from aws-crt-ffi

src/AWS/CRT/Internal/Encoding.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace AWS\CRT\Internal;
+
+final class Encoding {
+    public static function readString(&$buffer) {
+        list($len, $str) = self::decodeString($buffer);
+        // Advance by sizeof(length) + strlen(str)
+        $buffer = substr($buffer, 4 + $len);
+        return $str;
+    }
+
+    public static function readStrings($buffer) {
+        $strings = [];
+        while (strlen($buffer)) {
+            $strings []= self::readString($buffer);
+        }
+        return $strings;
+    }
+
+    public static function decodeString($buffer) {
+        $len = unpack("N", $buffer)[1];
+        $buffer = substr($buffer, 4);
+        $str = unpack("a${len}", $buffer)[1];
+        return [$len, $str];
+    }
+
+    public static function encodeString($str) {
+        return pack("Na*", strlen($str), $str);
+    }
+}