Use sigPrec in more places in Convert and HsUtils

Trac #16183 (closed) was caused by TH conversion (in Convert) not properly inserting parentheses around occurrences of explicit signatures where appropriate, such as in applications, function types, and type family equations. Solution: use parenthesizeHsType sigPrec in these places. While I was in town, I also updated nlHsFunTy to do the same thing.