Replace ol.Coordinate with Array.<number>

This issue is a question:

Should we replace `ol.Coordinate` with `Array.<number>`?

This is quite a major change, but has many advantages:

* Quicker to type (`[0, 0]` instead of `new ol.Coordinate(0, 0)`)
* Easily extensible to 3 or more dimensions
* `Array`s are generally more efficiently implemented by JS runtimes than `Object`s
* Increased opportunity for object re-use, leading to reduced GC load
* Easier interoperability with the packed arrays of vertices used by WebGL (see the `webgl-vector` branch)