ghc-internal: add MonadFix instance for (,)

Closes #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.

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• add a [testcase to the testsuite][adding test].
• updates the users guide if applicable
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user-facing