The root cause of #8450 is that the new Template Haskell story, with
the renamer doing more of the work of Template Haskell, wasn't dealing
correctly with the keepAlive problem.  Consider
    g = ..blah...
    f = [| g |]
Then f's RHS refers to g's name but not to g, so g was being discarded
as dead code.

Fixing this sucked me into a deep swamp of understanding how all the moving
parts of hte new Template Haskell fit together, leading to a large collection
of related changes and better documentation.  Specifically:

* Instead of putting the TH level of a binder in the LocalRdrEnv, there
  is now a separate field
      tcl_th_bndrs :: NameEnv (TopLevelFlag, ThLevel)
  in the TcLclEnv, which records for each binder
     a) whether it is syntactically a top-level binder or not
     b) its TH level
  This deals uniformly with top-level and non-top-level binders, which was
  previously dealt with via greviously-delicate meddling with Internal and
  External Names.  Much better.

* As a result I could remove the tct_level field of ATcId.

* There are consequential changes in TcEnv too, which must also extend the
  level bindings.  Again, more clarity.

  I renamed TcEnv.tcExtendTcTyThingEnv to tcExtendKindEnv2, since it's only used
  during kind inference, for (AThing kind) and APromotionErr; and that is
  relevant to whether we want to extend the tcl_th_bndrs field (no).

* I de-crufted the code in RnEnv.extendGlobalRdrEnv, by getting rid of the
  qual_gre code which said "Seems like 5 times as much work as it deserves!".
  Instead, RdrName.pickGREs makes the Internal names shadow External ones.

* I moved the checkThLocalName cross-stage test to finishHsVar; previously
  we weren't doing the test at all in the OpApp case!

* Quite a few changes (shortening the code) in the cross-stage checking code
  in TcExpr and RnSplice, notably to move the keepAlive call to the renamer

One leftover piece:

* In TcEnv I removed tcExtendGhciEnv and refactored
  tcExtendGlobalTyVars; this is really related to the next commit, but
  it was too hard to disentangle.