Initial implementation of converting to ContentProviderOperation

Initial implementation of converting ProviderActions to
ContentProviderOperations. Some things still need to be implemented:

- assert query?
- back refs?
- ...?
Make CrudHandlers support applyBatch
Make ConvertibleToOperation expose toContentProviderOperationBuilder()
Batcher - Support for batching ProviderActions.
simplify interface - remove that awkard batch() method
Only Inserts can be used as back references
Crash if referencing wrong Insert in back refs
Make API even better - expose just the one append() type.

Remove appendWithBackRef for cleaner api.
Make BackRefBuilder the top level class