I've spent quite a bit of time giving unique labels to my `pprTrace`
calls and then trying to intuit where the function is called from.
Thanks to the new implicit parameter CallStack functionality I don't
have to do that anymore.

Test Plan: harbormaster

Reviewers: austin, simonmar, bgamari

Reviewed By: simonmar, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1440