3e745313

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Mutation: fix SwitchFunctionArguments double-firing on parenthesised applications

collectApp peeled an enclosing HsPar, so a parenthesised application like
succ (div a b) was matched both at the parenthesis node and at the inner
application node, emitting the same swap mutation twice.  The walker
already recurses into an HsPar with instrumentLExpr (which re-runs every
operator), so stopping at the HsPar leaves the inner application as the
single firing site.

Adds Example.SwitchArgsLib.parenDivide as a regression case (its golden
now shows exactly one swap) plus a killing spec.