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.