Some tidying up of type pretty-printing
Triggered by the changes in #13677, I ended up doing a bit of refactoring in type pretty-printing. * We were using TyOpPrec and FunPrec rather inconsitently, so I made it consisent. * That exposed the fact that we were a bit undecided about whether to print a + b -> c + d vs (a+b) -> (c+d) and similarly a ~ [b] => blah vs (a ~ [b]) => blah I decided to make TyOpPrec and FunPrec compare equal (in BasicTypes), so (->) is treated as equal precedence with other type operators, so you get the unambiguous forms above, even though they have more parens. We could readily reverse this decision. See Note [Type operator precedence] in BasicTypes * I fixed a bug in pretty-printing of HsType where some parens were omitted by mistake.