Moving hadrian to submodule

Hello everyone, A bit over a month ago we merged hadrian into the ghc tree as a subtree. Unfortunately, those working on Hadrian have found the subtree mechanism to provide a rather poor developer experience. Consequently, today I will be ripping out the subtree and replacing it with a submodule. After pulling the commit performing this change you will likely need to do the following to emplace the new submodule, $ git submodule update --init $ git -C hadrian checkout . Cheers, - Ben