By having the `pp.proofs` feature use `⋯` when omitting proofs, when
users copy/paste terms from the InfoView the elaborator can give an
error message explaining why the term cannot be elaborated.
Also adds `pp.proofs.threshold` option to allow users to pretty print
shallow proof terms. By default, only atomic proof terms are pretty
printed.
This adjustment was suggested in PR #3201, which added `⋯` and the
related `pp.deepTerms` option.
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>
