Create Admin controller

[ethanfrey]

Closes #181

• Create new package for controllers, outside of cw0
• Add Admin controller here
• Integrate it into cw4-group
• Integrate it into cw4-stake

Follow-Up:

• Port Hooks/Claim controllers to the same pattern (and move packages) Hooks controller #195
• More some structs into cw0, so they can be imported by eg. cw4?

[ethanfrey]

@maurolacy curious what you think of this (just focus on cw4-group/srccontracts.rs and controllers/src/admin.rs.

Admin is a trivial example and maybe easy to cut and paste, but I was looking for a format to move these controllers, for maximum composability. So I can port hooks, claims, etc into this format.

[maurolacy]

Hi, OK, will take a look tomorrow.

Making slow progress with #163 ... let's talk tomorrow, as I'm almost there with the impl, and have some questions for you. Good thing is that I now have a much better grasp at storage-plus (Map, Prefix and the like).

[ethanfrey]

Nice. Yeah, storage-plus is a good piece to master and review by usage.

Good to review the api first with real usages. Then the implementation. I am sure there are ways to clean it up. Especially all the length prefixing calls. But best to actually work on the code.

I hope the Key trait makes sense (and you see how this allows us to naively construct indexes)