extensible-0.5: Extensible, efficient, optics-friendly data types and effects

Index - <

<!Data.Extensible.Product, Data.Extensible
<%Data.Extensible.Plain
<%|Data.Extensible.Plain
<:Data.Extensible.Product, Data.Extensible
<:|Data.Extensible.Sum, Data.Extensible
<@=>Data.Extensible.Field, Data.Extensible