Add a WrappingClock

The `WrappingClock` allows using an object with a `now()` method
returning a `DateTimeImmutable` object as a "real" clock.

examples/wrapping_clock.php

@@ -0,0 +1,28 @@
+<?php
+
+require __DIR__.'/../vendor/autoload.php';
+
+use Beste\Clock\WrappingClock;
+
+// Create a frozen $now so that we can test the wrapping clock.
+$now = new DateTimeImmutable('2012-04-24 12:00:00');
+
+// Create an object that is NOT a clock, but has a now() method returning the frozen $now.
+$clock = new class($now) {
+    private \DateTimeImmutable $now;
+
+    public function __construct(\DateTimeImmutable $now)
+    {
+        $this->now = $now;
+    }
+
+    public function now(): \DateTimeImmutable
+    {
+        return $this->now;
+    }
+};
+
+// We can now wrap the object in a clock.
+$wrappedClock = WrappingClock::wrapping($clock);
+
+assert($now->format(DATE_ATOM) === $wrappedClock->now()->format(DATE_ATOM));