Factored some labelled/singleton related macros for later use by Witness and HList/Coproduct type selectors
Added Witness type selector
Allowed literal types in Record/Union type selectors, and empty records/unions
Added HList and Coproduct type selectors
Removed unused argument of ZipWithKeys dependent functions
Added argument-less zipWithKeys overloads
Merge pull request #292 from alexarchambault/topic/dynmacroszipwithkeys

Type selectors, zipWithKeys enhancements