Update coding style to include PSR-12, among other options

This also includes heavy use of slevomat/coding-standard to apply
various checks to the code, based on maintainer (me) preference.

tests/Generator/RandomGeneratorFactoryTest.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Ramsey\Uuid\Test\Generator;
 
 use Ramsey\Uuid\Generator\RandomBytesGenerator;