Tidy modernize

Please have a look the the individual [tidy] commits. I think they're all debatable/optional, but I especially like 'concat-nested-namespaces', 'modernize-use-using' and 'readability-else-after-return'. There's a couple more 'readability-*' that sound interesting, however tidy errors out with those enabled. In theory we could make all the transformations into a CI job to check compliance but it's somewhat slow (think compile + headercheck * 1.5) and not 100% error proof.