Expose dual so that we can author fp-ts style data structures

[anilanar]

As the title says. Otherwise, will have to copy/paste dual.

E.g. libraries such as io-ts, fp-ts-contrib probably would like to use dual. I would like to use dual in our closed source codebase.

[gcanti]

Before effect@RC1 we can only backport features that we consider stable enough (like flatMap for example). dual changed over time and I'm not sure it won't change again before RC1, so better to keep it internal for now