Clean up TcHsSyn.zonkEnv

Triggered by Trac #15552, I'd been looking at ZonkEnv in TcHsSyn.

This patch does some minor refactoring

* Make ZonkEnv into a record with named fields, and use them.
  (I'm planning to add a new field, for TyCons, so this prepares
  the way.)

* Replace UnboundTyVarZonker (a higer order function) with the
  simpler and more self-descriptive ZonkFlexi data type, below.
 It's just much more perspicuous and direct, and (I suspect)
 a tiny bit faster too -- no unknown function calls.

data ZonkFlexi   -- See Note [Un-unified unification variables]
  = DefaultFlexi    -- Default unbound unificaiton variables to Any
  | SkolemiseFlexi  -- Skolemise unbound unification variables
                    -- See Note [Zonking the LHS of a RULE]
  | RuntimeUnkFlexi -- Used in the GHCi debugger

There was one knock-on effect in the GHCi debugger -- the
RuntimeUnkFlexi case.  Somehow previously, these RuntimeUnk
variables were sometimes getting SystemNames (and hence
printed as 'a0', 'a1', etc) and sometimes not (and hence
printed as 'a', 'b' etc).  I'm not sure precisely why, but
the new behaviour seems more uniform, so I just accepted the
(small) renaming wibbles in some ghci.debugger tests.

I had a quick look at perf: any changes are tiny.

1 files changed, 3 insertions, 12 deletions

diff --git a/compiler/ghci/RtClosureInspect.hs b/compiler/ghci/RtClosureInspect.hs
index 95c2e37136..34a0098bda 100644
--- a/compiler/ghci/RtClosureInspect.hs
+++ b/compiler/ghci/RtClosureInspect.hs
@@ -39,7 +39,7 @@ import Var
 import TcRnMonad
 import TcType
 import TcMType
-import TcHsSyn ( zonkTcTypeToType, mkEmptyZonkEnv )
+import TcHsSyn ( zonkTcTypeToType, mkEmptyZonkEnv, ZonkFlexi( RuntimeUnkFlexi ) )
 import TcUnify
 import TcEnv
 
@@ -1257,17 +1257,8 @@ zonkTerm = foldTermM (TermFoldM
 
 zonkRttiType :: TcType -> TcM Type
 -- Zonk the type, replacing any unbound Meta tyvars
--- by skolems, safely out of Meta-tyvar-land
-zonkRttiType = zonkTcTypeToType (mkEmptyZonkEnv zonk_unbound_meta)
-  where
-    zonk_unbound_meta tv
-      = ASSERT( isTcTyVar tv )
-        do { tv' <- skolemiseRuntimeUnk tv
-             -- This is where RuntimeUnks are born:
-             -- otherwise-unconstrained unification variables are
-             -- turned into RuntimeUnks as they leave the
-             -- typechecker's monad
-           ; return (mkTyVarTy tv') }
+-- by RuntimeUnk skolems, safely out of Meta-tyvar-land
+zonkRttiType = zonkTcTypeToType (mkEmptyZonkEnv RuntimeUnkFlexi)
 
 --------------------------------------------------------------------------------
 -- Restore Class predicates out of a representation type