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Arity analysis used to propagate optimistic arity types during fixed-point interation through the `ArityEnv`'s `ae_cheap_fun` field, which is like `GHC.Core.Utils.exprIsCheap`, but also considers the current iteration's optimistic arity, for the binder in question only. In #18793, we have seen that this is a problematic design, because it doesn't allow us to look through PAP bindings of that binder. Hence this patch refactors to a more traditional form with an explicit signature environment, in which we record the optimistic `ArityType` of the binder in question (and at the moment is the *only* binder that is recorded in the arity environment).
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