Provide a default string representation

Author: thekid

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

@@ -9,7 +9,10 @@ public abstract function compress($data);
 
   public abstract function decompress($compressed);
 
+  public function toString() { return nameof($this).'#'.$this->id; }
+
   public function hashCode() { return 'C'.$this->id; }
 
   public function compareTo($value) { return $value instanceof self ? $this->id <=> $value->id : 1; }
+
 }

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

@@ -2,15 +2,9 @@
 
 class Zstd extends Compressor {
   public $id= 3;
-  public $level;
-
-  /** @param int $level */
-  public function __construct($level= -1) {
-    $this->level= $level;
-  }
 
   public function compress($data) {
-    return zstd_compress($data, $this->level);
+    return zstd_compress($data);
   }
 
   public function decompress($compressed) {