Add Zstd implementation

Based on https://github.com/kjdev/php-ext-zstd

Author: thekid

src/main/php/com/mongodb/io/Zstd.class.php

@@ -0,0 +1,19 @@
+<?php namespace com\mongodb\io;
+
+class Zstd extends Compressor {
+  public $id= 2;
+  public $level;
+
+  /** @param int $level */
+  public function __construct($level= -1) {
+    $this->level= $level;
+  }
+
+  public function compress($data) {
+    return zstd_compress($data, $this->level);
+  }
+
+  public function decompress($compressed) {
+    return zstd_uncompress($compressed);
+  }
+}