Suggest adding Proxy in ambiguous type error message

The error message for an ambiguous type suggests enabling AllowAmbiguousTypes. However, this forces users to rely on TypeApplications and in some cases it's not enough (https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/ambiguous_types.html)

This adds a suggestion to use Proxy. There's a larger accepted GHC proposal but at least this is an incremental step.

Minor cleanup: there's no point in checking allow_ambiguous in the message, since it is used inside of an unless allow_ambiguous block.