Note [runRW magic]

Some definitions, for instance @runST@, must have careful control over float out
of the bindings in their body. Consider this use of @runST@,

    f x = runST ( \ s -> let (a, s')  = newArray# 100 [] s
                             (_, s'') = fill_in_array_or_something a x s'
                         in freezeArray# a s'' )

If we inline @runST@, we'll get:

    f x = let (a, s')  = newArray# 100 [] realWorld#{-NB