Generic builder for `Variant` based types

[paluh]

I'm not sure if we want to bring purescript-variant dependency here so I've created separate repo with these two helpers:

https://github.com/paluh/purescript-routing-duplex-variant/blob/master/src/Routing/Duplex/Generic/Variant.purs#L52

If you like them and think they should be included here I would be more than happy to open a PR. If not I would document them and publish as a separate lib and.. be happy too :-P

Thanks!

[natefaubion]

I would like to support variant, but I would prefer to not have a dependency on heterogeneous to accomplish it.

[paluh]

(...) I would prefer to not have a dependency on heterogeneous (...)

Interesting. May I ask why?

By the way - here is a version with manual folding / mapping:

https://github.com/paluh/purescript-routing-duplex-variant/blob/master/src/Routing/Duplex/Generic/Variant.purs

Do you think it is good enough for a PR and review?