Limited feature exports

We should not export the following:

 * [ ] `ol.Feature.prototype.set`
 * [ ] `ol.Feature.prototype.setFeatureId`
 * [ ] `ol.Feature.prototype.setGeometry`
 * [ ] `ol.Feature.prototype.setSymbolizers`

This will leave us room to change how feature modification is handled (so we don't have to register listeners on individual features to detect changes).