Moving hadrian to submodule

[Ara Adkins]

Sounds good! Hopefully this doesn’t cause a flood of commit messages. 

_ara

> On 8 Dec 2017, at 18:50, Ben Gamari <ben at well-typed.com> wrote:
> 
> 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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