Make internal proj constructors private

The `ol.proj.EPSG3857` and `ol.proj.EPSG4326` constructors were not used outside the modules where they are defined.  The change here renames these constructors to make them private.  Since we do use other constants in the `ol.proj.EPSG3857` and `ol.proj.EPSG4326` namespaces, those are still provided.

We are left with the `ol.proj.CH` projection subclass, which is not used in the library and is a potential candidate for removal.  I could add a commit that removes [`chprojection.js`](https://github.com/tschaub/ol3/blob/521cbfb1b0c4cebe6affda350cefae8e27160209/src/ol/proj/chprojection.js), but will wait to hear comments from anyone else who might see utility in it (@fredj?).

This is an alternative to #2328.