Work in progress on runRW#wip/T17760-runRW

This is a proof of concept, in progress. It treats
    runRW# (\s. e)
specially in three ways

* In the simplifier, we transform
    K[ runRW# rr ty (\s. body) ]
    -->  runRW rr' ty' (\s. K[ body ])
  where K is a context

* In Lint, join points are allowed to occur inside the continuation.
    join j x = rhs
    in runRW# (\s. case ... of
                     A -> j 1
                     B -> ...
                     C -> J 2)
  Very much as they can occur in other join points.

* In OccurAnal, we infer join points using the same rule

We get much beter optimisation as a result.

Still not finished.  E.g. Float out may take
    runST# (\s. e)
and float that lambda out.  But really we want to keep
that lambda in runST#'s argument, otherwise things that
were join point might stop being so.

But it's a start

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