missing visibility annotations in IO Types

[pixelzoom]

Lots of missing visibility annotations in IO Types and IOImpl inner classes. Recommended to review and fix.

[pixelzoom]

I fixed a few in the above commits, but not comprehensive.

[samreid]

This will no longer be necessary after we adopt the proposal in #211, on hold until that issue is further along.

[samreid]

IOType now provides these methods as passed-in options, not as overridden methods that require JSDoc. As we move methods like toStateObject to the core types, we will need to remember to add visibility annotations for those, but I think lint rules will help us remember. This issue seems ready to close.