More faff to get GHCi's top-level environment right

This fixes #8540 (again), and simplifies matters a bit more. In
particular, I got rid of ic_sys_vars altogether.  Mostly they can just
go in ic_tythings, apart from dfuns, which are readily gettable from
the instances anyway.

See documentation in Note [Initialising the type environment for GHCi]
in TcEnv.