Use unicode arrows with -fprint-unicode-syntax

See #8959 (closed), this is one more place where we can pretty-print Unicode syntax.

Test Plan: validate

Reviewers: nomeata, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #8959 (closed)

Differential Revision: https://phabricator.haskell.org/D5439