Tidy: Add flag to expose unfoldings if they take dictionary arguments.

Add the flag `-fexpose-overloaded-unfoldings` to be able to control this
behaviour.

For ghc's boot libraries file size grew by less than 1% when it was
enabled. However I refrained from enabling it by default for now.

I've also added a section on specialization more broadly to the users
guide.

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Metric Decrease:
    MultiLayerModulesTH_OneShot
Metric Increase:
    T12425
    T13386
    hard_hole_fits
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