Fix #16518 with some more kind-splitting smarts

This patch corrects two simple oversights that led to #16518 (closed):

1. HsUtils.typeToLHsType was taking visibility into account in the TyConApp case, but not the AppTy case. I've factored out the visibility-related logic into its own go_app function and now invoke go_app from both the TyConApp and AppTy cases.
2. Type.fun_kind_arg_flags did not properly split kinds with nested foralls, such as (forall k. k -> Type) -> (forall k. k -> Type). This was simply because fun_kind_arg_flags's FunTy case always bailed out and assumed all subsequent arguments were Required, which clearly isn't the case for nested foralls. I tweaked the FunTy case to recur on the result kind.