Moving hadrian to submodule

[Ben Gamari]

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
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