Add show `@printer` support for polymorphic variants

[sim642]

This missing support is listed as one of the motivations for ppx_deriving_variant_string:

printer is only supported in polyvars for some reason, but not on regular variants, which was a feature we wanted to have

I quickly implemented this and only then discovered that #267 already exists. The difference is that this is for the migrated ppxlib attributes and does use wrap_printer.