Expose the getUid function

It is often useful in application code to have a way to uniquely identify
and object.

Exposing the OpenLayers mechanism allows for best performance and avoid
unneccessary dirt.

src/ol/util.js

@@ -44,6 +44,7 @@ let uidCounter_ = 0;
  *
  * @param {Object} obj The object to get the unique ID for.
  * @return {number} The unique ID for the object.
+ * @api
  */
 export function getUid(obj) {
   return obj.ol_uid || (obj.ol_uid = ++uidCounter_);