Arity: Refactor fixed-point iteration in GHC.Core.Opt.Arity

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).