With `QuantifiedConstraints`, `forall`s can appear in more
nested positions than they could before, but `Convert` and the TH
pretty-printer were failing to take this into account. On the
`Convert` side, this is fixed by using a `parenthesizeHsContext`
to parenthesize singleton quantified constraints that appear to the
left of a `=>`. (A similar fix is applied to the TH pretty-printer.)

Test Plan: make test TEST=T15738

Reviewers: goldfire, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15738

Differential Revision: https://phabricator.haskell.org/D5222