Make Data.Functor.Identity trustworthy again

Alas `{-# LANGUAGE Safe #-}` can't be used since `Data.Coerce` isn't "safe".
However, we use `coerce` just as an optimisation
(see also 4ba884bdd3a9521ea92fcda8f601a7d0f3537bc1 which broke the
safe-inferred status of `Data.Functor.Identity`), so this module at least
deserves `{-# LANGUAGE Trustworthy #-}`.

NOTE: `Data.Functor.Identity` was added to `base` in the context of #9664

Reviewed By: luite

Differential Revision: https://phabricator.haskell.org/D507