Ben Gamari authored Feb 11, 2017 and Ben Gamari committed Feb 11, 2017

Previously it was quite difficult to follow the dataflow through this
file due to global mutation and rather non-descriptive types.

This is a cleanup in preparation for factoring out the report-generating
logic, which is itself in preparation for somedayteaching the profiler
to produce more machine-readable reports (JSON perhaps?).

Test Plan: Validate

Reviewers: austin, erikd, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3096