Minimize method count for nanoproto.

- Migrates getCachedSize to the MessageNano parent class to save one method per message.
- Create ExtendableMessageNano parent class for protos with extensions, this saves the
getExtension and setExtension methods on the relevant messages.
- getSerializedSize's default case (with no fields to serialize) also migrate to the
parent class, which saves methods on empty messages.
- Container classes become interfaces to save the constructor.

Change-Id: I81f1a1b6d6a660096835e9df3ea20456655aab4a