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, 1 insertions, 1 deletions

diff --git a/testsuite/tests/ghci.debugger/scripts/print019.stdout b/testsuite/tests/ghci.debugger/scripts/print019.stdout
index aea474019d..932fd540c3 100644
--- a/testsuite/tests/ghci.debugger/scripts/print019.stdout
+++ b/testsuite/tests/ghci.debugger/scripts/print019.stdout
@@ -1,4 +1,4 @@
-a = O (_t1::a)
+a = O (_t1::a0)
 b = O (_t2::a1)
 ()
 _t1 = 'a'