Need to support schemas with union types

[nadeemabdulhamid]

For example, substructures may differ in data from
"http://www.google.com/trends/topcharts/category?ajax=1&geo=US&date=201310"

[nadeemabdulhamid]

Although, you could still fetch from them, they don't show up in the manifest and it's sort of clunky:

(require sinbad)

(define ds
  (sail-to "http://www.google.com/trends/topcharts/category?ajax=1&geo=US&date=201710"
           (format "json")
           (manifest)
           (load)))

(fetch ds "data/chartList/trendingChart/visibleName")
(fetch ds "data/chartList/topChart/entityList/title")