Authored by dfeuer on Jul 21 2018, 2:45 PM.

Description

Harden fixST

Trac Trac #15349 reveals that lazy blackholing can cause trouble for
fixST much like it can for fixIO. Make fixST work just
like fixIO.

Reviewers: simonmar, hvr, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: Trac #15349

Differential Revision: https://phabricator.haskell.org/D4948

(cherry picked from commit 5a49651f3161473b383ec497af38e9daa022b9ac)