There are also a few general corrections to PHPDocs that I noticed along
the way (e.g. adding `|null` when the method is returning a null value.
There are some cases where either the return type or the whole PHPDoc
was duplicated from the parent class - in those cases I've simply
removed the duplication.
* API Implement InheritedPermission calculator
* API Rename RootPermissions to DefaultPermissionChecker
API Refactor inherited permission fields into InheritedPermissionExtension
API Introduce PermissionChecker interface
