Marked minimal parts of UuidInterface and Uuid named constructors as pure/immutable

This change is the minimum requirement to make the static analysis suite pass checks
on immutability/purity invariants of a `Uuid` and `UuidInterface`: it doesn't ensure
that internals of `Uuid` are pure. That will need deprecation of `Uuid::setFactory()`,
as well as marking any types referenced in private state as `@psalm-immutable` too.

That's out of scope for the current patch, which provides a good 80/20 split on purity
invariants for most (even advanced) UUID downstream users.

src/UuidInterface.php

@@ -22,6 +22,8 @@ use Serializable;
 /**
  * A UUID is a universally unique identifier adhering to an agreed-upon
  * representation format and standard for generation
+ *
+ * @psalm-immutable
  */
 interface UuidInterface extends JsonSerializable, Serializable
 {