Fix ambiguous use of OVERLAPPING instances in genericShrink

[MaximilianAlgehed]

As per the following documentation for how OVERLAPPING resolves (that I'm 99% sure is the same across all GHC versions) https://downloads.haskell.org/~ghc/7.10.1/docs/html/users_guide/type-class-extensions.html#instance-overlap
the way that ghc resolves OVERLAPPING pragmas is that an instance that is OVERLAPPING will be chosen over one that isn't. It's not clear why the old code ever worked, possibly because GSubtermsIncl (K1 i a) a is a substitution of GSubtermsIncl (K1 i a) b...

Anway, with this change it's explicit that the GSubtermsIncl (K1 i a) a instance will be chosen over the GSubtermsIncl (K1 i a) b instance.

[MaximilianAlgehed]

While this does not alleviate the concern in #84 about GHC picking the "shrink to empty list" instance when it can't conclude that two types are equal, it does make the rules for choosing instances more clear. I'm not convinced that the issue raised in #84 is real, however.

[Lysxia]

It's not clear why the old code ever worked, possibly because GSubtermsIncl (K1 i a) a is a substitution of GSubtermsIncl (K1 i a) b...

Yes, that's why. The extra OVERLAPPING annotation was not ambiguous, but unnecessary.