specialize Js.Dict.fromArray

part of #734 
Currently there is no way to generate JSON object below: 

```
{ "key1" : value1,
  "key2" : value2}
```

the closest one is `Js.Dict.fromArray : (string * Js.Json.t) array -> Js.Json.t Js.Dict.t)` but it results in a runtime call.

```
Js.Dict.fromArray [ "x", x ; "z", z]
```
Generated JS should be 
```
{ "x" :  x ,
   "z" : z}
```
