Summary:
`nlHsFunTy` wasn't parenthesizing its arguments at all,
which led to `-ddump-deriv` producing incorrectly parenthesized
types (since it uses `nlHsFunTy` to construct those types), as
demonstrated in #15307. Fix this by changing `nlHsFunTy` to add
parentheses à la `ppr_ty`: always parenthesizing the argument type
with function precedence, and recursively processing the result type,
adding parentheses for each function type it encounters.

Test Plan: make test TEST=T14578

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15307

Differential Revision: https://phabricator.haskell.org/D4890