Drop any of these functions that we are not currently using or plan to use soon.  We often introduce helper types like this without introducing the noise (code, test, doc) of all these functions that may or may never get used.  Before, I suggested to simply mirror [Interval](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Core/Interval.js), which is just data.

Also, do not include the prototype functions for types like these.  See #1082.