[4.x] Use ::class constants where possible

[mhujer]

It helps with refactorings and static analysis, because you can easily tell if the class is referenced or not.

[mabar]

@mhujer It is possible to add ::class constants everywhere, as it depends only on use statements. Autoloading is not triggered and existence of these classes is not required.

Try it yourself:

use Foo\Bar;
echo Bar\Baz::class;

[ramsey]

The reason I didn't use ::class in pre-4.x versions of the library was because ::class was only introduced in PHP 5.5, and this library needed to work in version 5.4. Since ramsey/uuid 4.x will no longer support PHP 5.x, this breaking change is acceptable.

Thanks!