Add pprSTrace for debugging with call stacks
Add pprSTrace for debugging with call stacks

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

mcschroeder/ghc,oldmanmike/ghc,gridaphobe/ghc,tjakway/ghcjvm,sgillespie/ghc,olsner/ghc,da-x/ghc,elieux/ghc,da-x/ghc,mettekou/ghc,snoyberg/ghc,ezyang/ghc,sgillespie/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,shlevy/ghc,da-x/ghc,ezyang/ghc,da-x/ghc,shlevy/ghc,olsner/ghc,vTurbine/ghc,mettekou/ghc,vikraman/ghc,mettekou/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,elieux/ghc,vikraman/ghc,vTurbine/ghc,mcschroeder/ghc,olsner/ghc,nushio3/ghc,mettekou/ghc,shlevy/ghc,oldmanmike/ghc,sdiehl/ghc,vTurbine/ghc,snoyberg/ghc,shlevy/ghc,sdiehl/ghc,gridaphobe/ghc,da-x/ghc,nushio3/ghc,olsner/ghc,vikraman/ghc,ezyang/ghc,nushio3/ghc,ezyang/ghc,oldmanmike/ghc,shlevy/ghc,GaloisInc/halvm-ghc,elieux/ghc,olsner/ghc,sdiehl/ghc,mcschroeder/ghc,vikraman/ghc,mettekou/ghc,snoyberg/ghc,olsner/ghc,sdiehl/ghc,oldmanmike/ghc,tjakway/ghcjvm,vTurbine/ghc,gridaphobe/ghc,ezyang/ghc,vikraman/ghc,tjakway/ghcjvm,vikraman/ghc,ezyang/ghc,snoyberg/ghc,nushio3/ghc,gridaphobe/ghc,sgillespie/ghc,nushio3/ghc,ezyang/ghc,mettekou/ghc,sdiehl/ghc,elieux/ghc,snoyberg/ghc,mcschroeder/ghc,vTurbine/ghc,oldmanmike/ghc,da-x/ghc,snoyberg/ghc,tjakway/ghcjvm,sgillespie/ghc,GaloisInc/halvm-ghc,vTurbine/ghc,sgillespie/ghc,mcschroeder/ghc,da-x/ghc,elieux/ghc,oldmanmike/ghc,GaloisInc/halvm-ghc,vTurbine/ghc,olsner/ghc,mettekou/ghc,elieux/ghc,nushio3/ghc,elieux/ghc,tjakway/ghcjvm,vikraman/ghc,sdiehl/ghc,shlevy/ghc,oldmanmike/ghc,gridaphobe/ghc,sgillespie/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,shlevy/ghc,tjakway/ghcjvm,nushio3/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,sdiehl/ghc,gridaphobe/ghc,sgillespie/ghc,gridaphobe/ghc