Restructuring hsSyn

[Shayan Najd]

Currently AST declarations, their relate utilities, and `Outputable`
instances are defined in the same files.
Does anyone object to moving `Outputable` instances to separate files?
The purpose is to gradually identify reusable functionalities, group them
together, polish them (e.g., remove some unnecessary dependencies), and
expose them to the end-users.
At this stage, I don't expect any changes outside hsSyn.

/Shayan
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