Ben Gamari authored Feb 23, 2017 and Ben Gamari committed Feb 23, 2017

This introduces a JSON output format for cost-centre profiler reports.
It's not clear whether this is really something we want to introduce
given that we may also move to a more Haskell-driven output pipeline in
the future, but I nevertheless found this helpful, so I thought I would
put it up.

Test Plan: Compile a program with `-prof -fprof-auto`; run with `+RTS
-pj`

Reviewers: austin, erikd, simonmar

Reviewed By: simonmar

Subscribers: duncan, maoe, thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D3132