Make NAI the default for coin symbols, deprecate and eventually drop support for legacy string names
I think we should probably switch to NAI form as "primary form" in next HF. On hived side, we can first assume NAI form, then fall back to legacy form if transaction fails for NAI. Later, in next HF after that, all apps should have switched to NAI, and we can drop support for legacy format. We can also potentially incentivize the switch by charging a higher RC cost for the legacy operation (if that isn't too much of a pain to implement).