Design for ARC20

[ghostant-1017]

[HarukaMa]

1. I believe there is already an upgradable program concept in Aleo, but it's not being further explored. Seems it is too early now to include that concept in the token standard. Personally, I'd like to see a minimal working token standard without all "fancy" features - they should be added later through updated ARCs or not included in the token standard at all.
2. It seems this program tries to contain many tokens within one program. Considering a program could only have one owner, I'd think it's a bad idea to let my token live in a program that I don't have the ultimate authority (it's unclear what the program owner could do in the future, but still). This approach might also limit what tokens could do; some tokens, while adhering to a common structure, might want to have their own features. IMO, a token standard should define a contract (what to have) for custom tokens, not a container for them.
3. The Credential concept seems strange to me - shouldn't it be stored in a public mapping instead of a private record? I believe this closely links to the point 2 above, and I think it's not a good approach for a token standard.

^{I might propose an ARC20 later as well}