Preserve parens in TH
Template Haskell supports parentheses in its AST. So does GHC. Yet GHC casually discards parentheses when desugaring TH quotes. We should preserve them.
I was originally thinking that this could get rid of
Note [Adding parens for splices] in Convert, but that processing would still be necessary if we are converting TH AST that did not come from a quote. Actually, it gets worse with this change, because we don't want to add redundant parens.
Still, I think preserving parens is the right thing to do here. It also makes it possible for a client to give special meaning to, say, nested parens: Perhaps
process [| f x |] and
process [| f ((x)) |] will behave differently. (Idiom brackets, anyone?)