tailRec instances aren't visible by default

[kubukoz]

As far as I know, import cats.syntax.all.* is now the recommended mega-import. However, it doesn't bring the instances for scala.util.control.TailCalls.TailRec - those still require import cats.instances.tailRec.*.

These instances should probably be included in the companion objects of the typeclasses themselves so that they're auto-discoverable like most other instances.