Add warnings to the cabal file

[aspiwack]

This helps, in particular, when developping with ghcid, which
otherwise doesn't raise warnings.

I've had to deactivate cabal format in the formatting script due to
the fact that it inlines common stanzas, which I'm using to ensure
consistent warning settings in every target. See
haskell/cabal#5734.

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I've written this while I was trying to do something smart with the
Elim and Make type classes. It didn't work. So there won't be a
followup PR, but this one is worth it on its own, I think.

[tbagrel1]

Well, I don't like the idea of removing cabal format (because it's nice to have alphabetically-sorted module names 😅 ); but adding common warnings seems to be more important. So let's do it (it's a pity that we don't have another solution at hand).

[aspiwack]

it's a pity that we don't have another solution at hand

Yet…

Let's be on the lookout.