This PR fixes the caching infrastructure for `whnf` and `isDefEq`,
ensuring the cache accounts for all relevant configuration flags. It
also cleans up the `WHNF.lean` module and improves the configuration of
`whnf`.
Switches from encoding `let_fun` using an annotated `(fun x : t => b) v`
expression to a function application `letFun v (fun x : t => b)`.
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Co-authored-by: Sebastian Ullrich <sebasti@nullri.ch>
