feat(data/equiv): pi_congr (#2204)
* feat(data/equiv): pi_congr
* docstrings
* change case for consistency
* tidying up
* switching names
* fixes
* Update src/data/equiv/basic.lean
* implicit arguments
Co-authored-by: Rob Lewis <Rob.y.lewis@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
