There are two related changes in here

a) Fix TcRnDriver.setInteractiveContext so that it initialises
   tcg_insts and tcg_fam_insts, as well as the corresponding
   tcg_inst_env and tcg_fam_inst_env fields.  Vital!

b) Fix FamInst.tcExtendLocalFamInstEnv for the GHCi case.
   It had a special HACK to allow family instances to be overridden
   in GHCi.  Fair enough, but
     * It was only affecting the tcg_fam_inst_env, not tcg_fam_insts
     * It overrode home-package instances as well as ones entered
       at the GHCi prompt, which probably wasn't intended.

   I fixed both of these, and also narrowed the scope of what it does
   to override only *identical* instances, rather than any ones that
   conflict.  If people yelp about this we can return to the question,
   but for now it narrows the range of the HACK