rts/Profiling: Kill a few globals and add consts

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