Change instances for Product and Sum to have (NFData (f a), NFData (g a)` constraints #93
Description
To be aligned with change Eq1 and Ord1 instances in base-4.18 (GHC-8.6). The #88 could done at the same time. The change in base is described in haskell/core-libraries-committee#10, AFAICT the rationale applies to deepseq as well.
cc @Icelandjack and @Ericson2314