Allow pushing of strict contexts into runRW# continuations

This contains my work on top of @simonpj's prototype 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 better optimisation as a result.