Use STATE_SCHEMA to limit IOType boilerplate

[zepumph]

Over in https://github.com/phetsims/phet-io/issues/1782, I finally got to cleaning up how stateSchema duplicates much of how applyState and toStateObject are most likely implemented.

In this sim specifically, I found 2 potential problems with just blowing away the toStateObject and the applyState methods on Bunny.js

• There was documentation associated with each, specific to parts of the implementation. Perhaps this could be moved to the declaration of the STATE_SCHEMA
• There is a side effect in applyState, to validateInstance. I went with using the new feature as a "library" function instead of using the framework in IOType to do it automatically (both are supported).

Note that the required calls are redundant because there is a lot of type validation that is done for anything in the state schema, both for applyState and toStateObject.

[zepumph]

@pixelzoom, would you please review the above commit. If you feel like the validateInstance isn't actually needed, then you can delete the entire applyState function and the IOType will automatically grab it from StateSchema.defaultApplyState.

[pixelzoom]

I'm going to leave the call to validateInstance in Bunny applyState. It's a useful additional layer of protection again PhET-iO regressions, and it's a pattern that I've used throughout Natural Selection. But thanks for bringing this to my attention. Closing.