Maybe you should add to doc information that developer should invalidate all users sessions.

This is a BC break for people extending theRole class and I'm not sure it is worth it:

Before:

1:{s:47:"\0Symfony\Component\Security\Core\Role\Role\0role";s:9:"ROLE_USER";}}

After:

16:{s:9:"ROLE_USER";}}

Length is 23 vs 79 for a role.

So, -1.

Closing it as it's not worth it to break BC for such a small benefit.