Refactor tuple constraints

Make tuple constraints be handled by a perfectly ordinary
type class, with the component constraints being the
superclasses:
    class (c1, c2) => (c2, c2)

This change was provoked by

  #10359  inability to re-use a given tuple
          constraint as a whole

  #9858   confusion between term tuples
          and constraint tuples

but it's generally a very nice simplification. We get rid of
 -  In Type, the TuplePred constructor of PredTree,
    and all the code that dealt with TuplePreds
 -  In TcEvidence, the constructors EvTupleMk, EvTupleSel

See Note [How tuples work] in TysWiredIn.

Of course, nothing is ever entirely simple. This one
proved quite fiddly.

- I did quite a bit of renaming, which makes this patch
  touch a lot of modules. In partiuclar tupleCon -> tupleDataCon.

- I made constraint tuples known-key rather than wired-in.
  This is different to boxed/unboxed tuples, but it proved
  awkward to have all the superclass selectors wired-in.
  Easier just to use the standard mechanims.

- While I was fiddling with known-key names, I split the TH Name
  definitions out of DsMeta into a new module THNames.  That meant
  that the known-key names can all be gathered in PrelInfo, without
  causing module loops.

- I found that the parser was parsing an import item like
      T( .. )
  as a *data constructor* T, and then using setRdrNameSpace to
  fix it.  Stupid!  So I changed the parser to parse a *type
  constructor* T, which means less use of setRdrNameSpace.

  I also improved setRdrNameSpace to behave better on Exact Names.
  Largely on priciple; I don't think it matters a lot.

- When compiling a data type declaration for a wired-in thing like
  tuples (,), or lists, we don't really need to look at the
  declaration.  We have the wired-in thing!  And not doing so avoids
  having to line up the uniques for data constructor workers etc.
  See Note [Declarations for wired-in things]

- I found that FunDeps.oclose wasn't taking superclasses into
  account; easily fixed.

- Some error message refactoring for invalid constraints in TcValidity

1 files changed, 12 insertions, 12 deletions

diff --git a/testsuite/tests/typecheck/should_fail/tcfail157.stderr b/testsuite/tests/typecheck/should_fail/tcfail157.stderr
index acdc7df8cf..113e0cc67e 100644
--- a/testsuite/tests/typecheck/should_fail/tcfail157.stderr
+++ b/testsuite/tests/typecheck/should_fail/tcfail157.stderr
@@ -1,12 +1,12 @@
-
-tcfail157.hs:27:10:
-    Variable ‘b’ occurs more often than in the instance head
-      in the constraint: E m a b
-    (Use UndecidableInstances to permit this)
-    In the instance declaration for ‘Foo m (a -> ())’
-
-tcfail157.hs:27:10:
-    Variable ‘b’ occurs more often than in the instance head
-      in the constraint: Foo m b
-    (Use UndecidableInstances to permit this)
-    In the instance declaration for ‘Foo m (a -> ())’
+

+tcfail157.hs:27:10: error:

+    Variable ‘b’ occurs more often

+      in the constraint ‘E m a b’ than in the instance head

+    (Use UndecidableInstances to permit this)

+    In the instance declaration for ‘Foo m (a -> ())’

+

+tcfail157.hs:27:10: error:

+    Variable ‘b’ occurs more often

+      in the constraint ‘Foo m b’ than in the instance head

+    (Use UndecidableInstances to permit this)

+    In the instance declaration for ‘Foo m (a -> ())’