Note [No eta-expansion in runRW#]

When we see `runRW# (\s. blah)` we must not attempt to eta-expand that
lambda.  Why not?  Because
* `blah` can mention join points bound outside the runRW#
* eta-expansion uses arityType, and
* `arityType` cannot cope with free join Ids:

So the simplifier spots the literal lambda, and simplifies inside it.
It's a very special lambda, because it is the one the OccAnal spots and
allows join points bound /outside/ to be called /inside/.

See Note [No free join points in arityType] in GHC.Core.Opt.Arity

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                Rewrite rules
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