ghc-internal: add MonadFix instance for (,)

Closes ghc/ghc#24288, implements CLC
proposal https://github.com/haskell/core-libraries-committee/issues/238.

Adds a MonadFix instance for tuples, permitting value recursion in the
"native" writer monad and bringing consistency with the existing
instance for transformers's WriterT (and, to a lesser extent, for Solo).

See the CLC thread for proofs of the MonadFix laws.

libraries/ghc-internal/src/Control/Monad/Fix.hs

@@ -30,7 +30,7 @@ module Control.Monad.Fix (
 import Data.Either
 import Data.Function ( fix )
 import Data.Maybe
-import Data.Monoid ( Dual(..), Sum(..), Product(..)
+import Data.Monoid ( Monoid, Dual(..), Sum(..), Product(..)
                    , First(..), Last(..), Alt(..), Ap(..) )
 import Data.Ord ( Down(..) )
 import GHC.Base ( Monad, NonEmpty(..), errorWithoutStackTrace, (.) )
@@ -72,6 +72,10 @@ instance MonadFix Solo where
     mfix f = let a = f (unSolo a) in a
              where unSolo (MkSolo x) = x
 
+-- | @since 4.20
+instance Monoid a => MonadFix ((,) a) where
+    mfix f = let a = f (snd a) in a
+
 -- | @since 2.01
 instance MonadFix Maybe where
     mfix f = let a = f (unJust a) in a