Ryan Scott authored May 16, 2019 and Marge Bot committed May 21, 2019

Most places where we convert contexts in `Convert` are actually in
positions that are to the left of some `=>`, such as in superclasses
and instance contexts. Accordingly, these contexts need to be
parenthesized at `funPrec`. To accomplish this, this patch changes
`cvtContext` to require a precedence argument for the purposes of
calling `parenthesizeHsContext` and adjusts all `cvtContext` call
sites accordingly.