Tracking ticket for using TemplateHaskell in GHC bootstrapping

Currently GHC has a policy of avoiding use of TH in its bootstrapping process. However, we suspect that on-going work in cross-compilation should address many of the technical barriers to using it. Moreover, TH can be a compelling alternative to Generic instance deriving in many cases. @mpickering and I have discussed options for achieving this, including possibly running splices on the host.