Support JSString in ChoiceString when compiling with ghcjs

[waern]

This avoids costly conversions if you're writing an SPA and have JSStrings in your state.

[meiersi-da]

Thanks for the PR. I wonder whether there wouldn't be the possibility for a version of the Text library that uses JSString internally, but is API compatible with the default text library. That would allow us to get rid of the #ifdefs.

[asayers]

And you'd use backpack to swap out the different implementations?

[meiersi-da]

Hmm..interesting. I was thinking about conditionals in the cabal file, which would probably be placed in a text frontend library that includes text-native or text-ghcjs depending on the compiler that it is compiled with.

[asayers]

Oh yes, that also sounds like it would work (and has the benefit of using technology which exists).