Issue #2483781 by borisson_, rbayliss, cilefen, mitalimehta, joshi.rohit100: Move cache contexts classes from \Drupal\Core\Cache to \Drupal\Core\Cache\Context

Issue #2432837 by Wim Leers, Fabianx: Make cache contexts hierarchical (e.g. 'user' is more specific than 'user.roles')