Add instances for GHC.Generics data types

I'm currently in a situation where I want to `rnf` a value from an upstream library, but it doesn't define `NFData` instances.

It does, however, define a `Generic` instance. If there were `NFData` instances for the data types defined in `GHC.Generics` (eg `U1` etc), I could write `rnf . GHC.Generics.from`. Alternatively, if `deepseq` exported its `grnf` definition, I could similarly write `grnf . GHC.Generics.from`.

Either of those would let me avoid something like an orphan instance etc. (Those can be harmless eg in exes or tests, but I try to avoid them in general.)

Are there pitfalls that would make exporting these instances/`grnf` a footgun for users, or would this merely provide users more options? Thanks.

Start by reviewing the deepseq API around NFData and grnf, then inspect the GHC.Generics types mentioned in the issue, including U1. Determine whether adding NFData instances or exporting grnf is safe and unambiguous; done means the maintainers have a decided approach and corresponding documented behavior.

Written by the indexing model from the issue text.