Add @api stable annotations for ol.Feature and the ol.geom namespace

This PR adds `@api stable` annotations to constructors and methods for `ol.Feature` and the `ol.geom` package.

Please review.
Yes, I meant everywhere (all three places).

I'm not totally hung up on this, but it struck me as the sort of extra abstraction that was more likely to confuse than help.  It is a little less typing in the code for us (42 characters saved in the whole lib by introducing `ol.geom.RawMultiPolygon`).  But people already see `ol.Coordinate` all over the API docs, and it would be one fewer abstraction to think about to see that geometries could be constructed with arrays of those (rather than [clicking through](http://ol3js.org/en/master/apidoc/ol.geom.Point.html) to learn that [`ol.geom.RawPoint`](http://ol3js.org/en/master/apidoc/ol.geom.html#RawPoint) is the same as [`ol.Coordinate`](http://ol3js.org/en/master/apidoc/ol.html#Coordinate)).

And then if there is some compelling reason for the extra types, I think `ol.geom.PointCoordinate` is nicer.