Cleanup of definitional modules

[L0neGamer]

I was going to set off on #459, but found it incredibly hard to navigate Test.QuickCheck.Arbitrary; typeclasses and instances are scatted across the two thousand lines of CPP'd instances, making it hard to track what is defined where.

I would suggest that we make two changes: firstly, move the classes into their own modules (Test.QuickCheck.Arbitrary.Class, Test.QuickCheck.Arbitrary1.Class, Test.QuickCheck.CoArbitrary.Class; or just move all the classes to a Test.QuickCheck.Arbitrary.Class); secondly, reorganise instance definitions either on a class by class basis (Test.QuickCheck.Arbitrary.Instances) or on a type origin by type origin basis (like in https://github.com/haskellari/qc-instances/tree/master/src/Test/QuickCheck/Instances, resulting in Test.QuickCheck.Instances.Semigroup and so on). This would result in many orphan instances, but those are easily resolved by exporting them from the same Test.QuickCheck.Arbitrary as before.

I'm happy to have a go at this, but thought I should ask before setting off.

I would also ask for guidance on what this breaking up and cleaning up should look like.

[L0neGamer]

Follow up; someone pointed out to me that orphan-containing modules slow down compilation, see this part of the user guide: https://downloads.haskell.org/ghc/latest/docs/users_guide/separate_compilation.html#orphan-modules-and-instance-declarations.

I therefore suggest just splitting the typeclasses into their own modules, trying to avoid having all of that mess in one module. Thinking about it, I don't think this is simply feasible because some arbitrary instances may rely on Arbitrary1 instances, but I haven't looked further right now.

[MaximilianAlgehed]

The issue with splitting all this up is indeed introducing a bunch of orphan instances and the resulting (more or less minor) compilation and organization headaches that come with this. Likewise, it may not be as straight-forward as you like to split e.g. Arbitrary, Arbitrary1, CoArbitrary, etc. as some instances introduce circular dependencies between all of them.

If you want to improve the situation in the code as it stands a welcome effort would be cleaning up the CPP and simplifying it - if possible.