Fix some PHP deprecation errors

Author: matthiasnoback

Error/ValidationErrorList.php

@@ -11,12 +11,12 @@ public function add(ValidationErrorInterface $error)
         $this->errors[] = $error;
     }
 
-    public function count()
+    public function count(): int
     {
         return count($this->errors);
     }
 
-    public function getIterator()
+    public function getIterator(): \ArrayIterator
     {
         return new \ArrayIterator($this->errors);
     }

Exception/InvalidExpressionException.php

@@ -12,7 +12,7 @@ public function __construct($expression, \Exception $exception)
                 $expression,
                 $exception->getMessage()
             ),
-            null,
+            0,
             $exception
         );
     }

Exception/InvalidExpressionSyntaxException.php

@@ -14,7 +14,7 @@ public function __construct($expression, SyntaxError $exception)
                 $expression,
                 $exception->getMessage()
             ),
-            null,
+            0,
             $exception
         );
     }