Add explicit Stringable interface to UuidInterface (#491)

Even tho, the `Stringable` interface is implicit on all objects containing the method `__toString`, it is recommended by PHP to explicitly setting that interface.

From php.net documentation:
> [...] Unlike most interfaces, **Stringable** is implicitly present on any class that has the magic `__toString()` method defined, although it can and should be declared explicitly.

Signed-off-by: Maximilian Bösing <2189546+boesing@users.noreply.github.com>

src/UuidInterface.php

@@ -19,6 +19,7 @@ use Ramsey\Uuid\Fields\FieldsInterface;
 use Ramsey\Uuid\Type\Hexadecimal;
 use Ramsey\Uuid\Type\Integer as IntegerObject;
 use Serializable;
+use Stringable;
 
 /**
  * A UUID is a universally unique identifier adhering to an agreed-upon
@@ -29,7 +30,8 @@ use Serializable;
 interface UuidInterface extends
     DeprecatedUuidInterface,
     JsonSerializable,
-    Serializable
+    Serializable,
+    Stringable
 {
     /**
      * Returns -1, 0, or 1 if the UUID is less than, equal to, or greater than