Add "to-array" & "to-object" to expresions

Nobody has claimed this yet.

## Motivation

As MVT spec does not allow passing arrays or objects as features property, the shortcut is to pass them serialized as string, and parse it later if needed.

The problem here is when trying to use one specific element of that specific property for filtering or styling, it looks back to the original info in the tile and it will find a string. So, no way.

## Proposal

Adding new casts to [expresions](https://www.mapbox.com/mapbox-gl-js/style-spec/#expressions-types)

* **to-object** (~ JSON.parse(value)):
* use: `["to-object", my_object_like_value ]`
* example:
```` javascript
fill-opacity:["get", "myopacity", ["to-object", ["get", "myproperty"]]]
````
* **to-array** (~ value.split(separator)):
* use: `["to-array", separator , my_array_like_value ]`
* example:
```` javascript
fill-opacity: ["number", ["at", 23, ["to-array", ",", ["get", "myproperty"]]]]
````

Start with the expressions type documentation linked in the issue and review the proposed to-object and to-array examples. Determine the intended inputs, outputs, and filtering or styling behavior for each cast, then add coverage for the documented cases and verify that serialized feature properties can be used as proposed.

Written by the indexing model from the issue text.