Draft: Define one-shot EtaReaderT, use it in IOEnv (#19380)
In #19380 we discuss how we can hide the Note [The one-shot state monad trick]
behind a re-usable set of monad transformers replicating ReaderT, StateT,
etc.
This patch introduces GHC.Utils.Monad.EtaReaderT and uses it to implement
IOEnv, the implementation of which no longer needs to be aware of the one-shot
monad trick.
A first step toward resolving #19380. The ultimate goal is to upstream
EtaReaderT back to transformers, but we can play around with it now in GHC
to experience the implications of using it.