This PR improves the `congr` tactic so that it can handle function
applications with fewer arguments than the arity of the head function.
This also fixes a bug where `congr` could not make progress with
`Set`-valued functions in Mathlib, since `Set` was being unfolded and
making such functions have an apparently higher arity.
This addresses issue #2128 for the `congr` tactic, but not `simp` and
others.
Although `HEq` was abbreviated as `≍` in #8503, many instances of the
form `HEq x y` still remain.
Therefore, I searched for occurrences of `HEq x y` using the regular
expression `(?<![A-Za-z/@]|``)HEq(?![A-Za-z.])` and replaced as many as
possible with the form `x ≍ y`.
