Support for boxes

I'm currently making a rest API that needs to take the cords to construct a bounding box for a query. It would be great if rust-postgis had types for Box2d and Box3d. This would allow me to do things like..

```sql
SELECT *
FROM table
WHERE $1 && table.geog;
```

```sql
SELECT *
FROM table
WHERE ST_Intersects($1, table.geog);
```