[#265] Make SuperComposition less brittle

[treeowl]

• Use overlapping instances instead of incoherent ones. Fixes #265.

• Make the first argument of ... a function unconditionally, before instance selection. This can theoretically improve inference slightly, though it probably doesn't have much impact in practice.

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[treeowl]

This change causes compilation failures in the benchmark suite. I tried to patch that up using an incoherent instance (to let type information flow from the result type to the second argument type):

instance {-# INCOHERENT #-} (SuperComposition x y d q, cd ~ (c -> d)) =>
         SuperComposition x y cd (c -> q) where
    (f ... g) c = (...) @x @y @d @q f (g c)
    {-# INLINE (...) #-}

For some reason, that leads to a reduction stack overflow in the benchmark, which I find extremely mysterious. It looks to me like at least one type argument gets smaller in each recursive call, and none get larger. What am I missing???

[treeowl]

Oh, of course, that particular instance is just wrong, because the arrows could come from the first argument. Whoops! I think it's okay to break this a little, in exchange for more stable behavior.

[treeowl]

Where should I put a migration guide?