either type: improve merge function

      name = "either";

      description = "${t1.description} or ${t2.description}";

      check = x: t1.check x || t2.check x;

      merge = mergeOneOption;

      merge = loc: defs:

        let

          defList = map (d: d.value) defs;

        in

          if   all (x: t1.check x) defList

               then t1.merge loc defs

          else if all (x: t2.check x) defList

               then t2.merge loc defs

          else mergeOneOption loc defs;

      typeMerge = f':

        let mt1 = t1.typeMerge (elemAt f'.wrapped 0).functor;

            mt2 = t2.typeMerge (elemAt f'.wrapped 1).functor;