Merge branch '5.1' into 5.2

* 5.1:
  Update .php_cs.dist
  Apply "visibility_required" CS rule to constants

Author: nicolas-grekas

PropertyPath.php

@@ -25,7 +25,7 @@ class PropertyPath implements \IteratorAggregate, PropertyPathInterface
     /**
      * Character used for separating between plural and singular of an element.
      */
-    const SINGULAR_SEPARATOR = '|';
+    public const SINGULAR_SEPARATOR = '|';
 
     /**
      * The elements of the property path.