This attribute, which was implemented in #3640, is applied to the
following structures: `Sigma`, `PSigma`, `PProd`, `And`, `Subtype`, and
`Fin`. These were given this attribute in Lean 3.
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.
