Improve performance of RenameMap in LowerTypes (backport #2233) (#2234)

* Improve performance of RenameMap in LowerTypes (#2233)

LowerTypes creates a lot of mappings for the RenameMap. The built-in
.distinct of renames becomes a performance program for designs with
deeply nested Aggregates. Because LowerTypes does not create duplicate
renames, it can safely eschew the safety of using .distinct via a
private internal API.

(cherry picked from commit e0844966

)

* Waive bincompat issue that isn't visible from Scala

Co-authored-by: Jack Koenig <koenig@sifive.com>