Douglas Wilson authored Jul 11, 2017 and Ben Gamari committed Jul 11, 2017

An additional stat is tracked per gc: par_balanced_copied This is the
the number of bytes copied by each gc thread under the balanced lmit,
which is simply (copied_bytes / num_gc_threads).  The stat is added to
all the appropriate GC structures, so is visible in the eventlog and in
GHC.Stats.

A note is added explaining how work balance is computed.

Remove some end of line whitespace

Test Plan:
./validate
experiment with the program attached to the ticket
examine code changes carefully

Reviewers: simonmar, austin, hvr, bgamari, erikd

Reviewed By: simonmar

Subscribers: Phyx, rwbarton, thomie

GHC Trac Issues: #13830

Differential Revision: https://phabricator.haskell.org/D3658