Generic-derived FromSql instance

[mitchellwrosen]

It would be great to be able to derive FromSql from Generic :)

[bergey]

I got a bit more info from Mitchell in DMs:

TH is always a pain to use, as it sometimes requires moving code around to satisfy various visibility checks (like how a module can’t refer to splices defined inside of itself), plus the annoying compile-time hit you take due to the conservative (and correct) assumption that a TH splice may perform arbitrary IO. So whenever you touch module A that is imported by module B, where B contains a benign $deriveFromSql, regardless of if the interface file could otherwise tell GHC it isn’t necessary to recompile B (because something unused by B was modified in A), B will be recompiled regardless.

This isn't a high priority for me personally, but I'd gratefully accept a PR.