json coercion from arrays to Sets (on the clj-side)

hi.

Json-coercion from arrays (`sequential?`) to clojure `Set`s.

Use case with set of enums. A Schema type:

```clojure
#{(s/enum :cheese :olives :ham :pepperoni :habanero)}})
```

with json-input:

```json
"["ham", "ham", "pepperoni"]"
```

gets into:
```clojure
#{:ham :pepperoni}
```

Tommi