Implement BoxedRep proposal

This implements the BoxedRep proposal, refacoring the `RuntimeRep`
hierarchy from:

```haskell
data RuntimeRep = LiftedPtrRep | UnliftedPtrRep | ...
```

to

```haskell
data RuntimeRep = BoxedRep Levity | ...
data Levity = Lifted | Unlifted
```

Closes #17526.

3 files changed, 19 insertions, 9 deletions

diff --git a/docs/users_guide/9.2.1-notes.rst b/docs/users_guide/9.2.1-notes.rst
index 283615b7a4..bfa8283092 100644
--- a/docs/users_guide/9.2.1-notes.rst
+++ b/docs/users_guide/9.2.1-notes.rst
@@ -13,6 +13,12 @@ Language
   <https://www.microsoft.com/en-us/research/publication/a-quick-look-at-impredicativity/>`__
   (Serrano et al, ICFP 2020).  More information here: :ref:`impredicative-polymorphism`.
   This replaces the old (undefined, flaky) behaviour of the :extension:`ImpredicativeTypes` extension.
+* The first stage of the `Pointer Rep Proposal`_ has been implemented. All
+  boxed types, both lifted and unlifted, now have representation kinds of
+  the shape ``BoxedRep r``. Code that references ``LiftedRep`` and ``UnliftedRep``
+  will need to be updated.
+
+.. _Pointer Rep Proposal: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0203-pointer-rep.rst
 
 * Kind inference for data/newtype instance declarations is slightly
   more restrictive than before.  See the user manual :ref:`kind-inference-data-family-instances`.
diff --git a/docs/users_guide/exts/levity_polymorphism.rst b/docs/users_guide/exts/levity_polymorphism.rst
index a65f878b41..80a544e54b 100644
--- a/docs/users_guide/exts/levity_polymorphism.rst
+++ b/docs/users_guide/exts/levity_polymorphism.rst
@@ -13,21 +13,25 @@ Here are the key definitions, all available from ``GHC.Exts``: ::
 
   TYPE :: RuntimeRep -> Type   -- highly magical, built into GHC
 
-  data RuntimeRep = LiftedRep     -- for things like `Int`
-                  | UnliftedRep   -- for things like `Array#`
-                  | IntRep        -- for `Int#`
+  data Levity = Lifted    -- for things like `Int`
+              | Unlifted  -- for things like `Array#`
+
+  data RuntimeRep = BoxedRep Levity  -- for anything represented by a GC-managed pointer
+                  | IntRep           -- for `Int#`
                   | TupleRep [RuntimeRep]  -- unboxed tuples, indexed by the representations of the elements
                   | SumRep [RuntimeRep]    -- unboxed sums, indexed by the representations of the disjuncts
                   | ...
 
+  type LiftedRep = BoxedRep Lifted
+
   type Type = TYPE LiftedRep    -- Type is just an ordinary type synonym
 
 The idea is that we have a new fundamental type constant ``TYPE``, which
 is parameterised by a ``RuntimeRep``. We thus get ``Int# :: TYPE 'IntRep``
-and ``Bool :: TYPE 'LiftedRep``. Anything with a type of the form
+and ``Bool :: TYPE LiftedRep``. Anything with a type of the form
 ``TYPE x`` can appear to either side of a function arrow ``->``. We can
 thus say that ``->`` has type
-``TYPE r1 -> TYPE r2 -> TYPE 'LiftedRep``. The result is always lifted
+``TYPE r1 -> TYPE r2 -> TYPE LiftedRep``. The result is always lifted
 because all functions are lifted in GHC.
 
 .. _levity-polymorphic-restrictions:
@@ -102,13 +106,13 @@ Printing levity-polymorphic types
     :category: verbosity
 
     Print ``RuntimeRep`` parameters as they appear; otherwise, they are
-    defaulted to ``'LiftedRep``.
+    defaulted to ``LiftedRep``.
 
 Most GHC users will not need to worry about levity polymorphism
 or unboxed types. For these users, seeing the levity polymorphism
 in the type of ``$`` is unhelpful. And thus, by default, it is suppressed,
-by supposing all type variables of type ``RuntimeRep`` to be ``'LiftedRep``
-when printing, and printing ``TYPE 'LiftedRep`` as ``Type`` (or ``*`` when
+by supposing all type variables of type ``RuntimeRep`` to be ``LiftedRep``
+when printing, and printing ``TYPE LiftedRep`` as ``Type`` (or ``*`` when
 :extension:`StarIsType` is on).
 
 Should you wish to see levity polymorphism in your types, enable
diff --git a/docs/users_guide/exts/typed_holes.rst b/docs/users_guide/exts/typed_holes.rst
index 170824ee4f..4fded59ae2 100644
--- a/docs/users_guide/exts/typed_holes.rst
+++ b/docs/users_guide/exts/typed_holes.rst
@@ -443,7 +443,7 @@ it will additionally offer up a list of refinement hole fits, in this case: ::
       with const @Integer @[Integer]
       where const :: forall a b. a -> b -> a
     ($) (_ :: [Integer] -> Integer)
-      with ($) @'GHC.Types.LiftedRep @[Integer] @Integer
+      with ($) @GHC.Types.LiftedRep @[Integer] @Integer
       where ($) :: forall a b. (a -> b) -> a -> b
     fail (_ :: String)
       with fail @((->) [Integer]) @Integer