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: #15349

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