Issue title: Question: Vector2d as a public property not showing in Unity inspector
 Issue body: Hi!
I would like to know if there any special reason for leaving struct without [System.Serializable]?
.. or this is just a bug? ;) 
(I remember that some time ago there were problems with using structs in Unity)

```
namespace Mapbox.Utils
{
    using System;
    using System.Globalization;

	[System.Serializable]
    public struct Vector2d
```

 Comments: 
 Comment 0: @lgzajac Just an oversight on our part--thanks for pointing it out! 
{'source': 'GithubIssues', 'repo_name_with_owner': 'mapbox/mapbox-unity-sdk', 'issue_id': 255951982, 'repo_stars': 682.0}