This PR makes UUIDs usable with the PHP DS extension's Map and Set classes. It's a new feature and shouldn't cause any BC breaks.

The Map and Set classes in the DS extension have extended support for objects implementing the Ds\Hashable interface (as keys in Map and values in Set). By default, values are made unique by testing for strict equality, which will consider two Uuid instances to be distinct even though they represent the same UUID; with this PR merged, Uuid instances would behave as expected with these classes.

The current implementation of AbstractUid already includes the equals() method defined in the interface; the PR adds the missing hash() method and a shim for the interface in case the extension isn't loaded. I've also added a test for the new hash() method, checking both the behaviour of the method itself and the correctness of that behaviour when used with the Ds\Set class (that part of the test only runs if the ds extension is loaded).

I've read through the contributing docs, but it's still my first time contributing to Symfony, I'm sure I've made mistakes - please don't be too harsh on me! 🙏 ❤️