contract: swap, pay, vote

[dckc]

The contracts are described in:

• https://github.com/Agoric/documentation/issues/940
• https://github.com/Agoric/documentation/issues/941
• https://github.com/Agoric/documentation/issues/942

For each:

• [x] contract
• [x] wallet style test to specify the interface with the front end
• [x] sketch of core eval code

We'll address deployment separately in...

• https://github.com/Agoric/dapp-agoric-basics/issues/15

cc @LuqiPan @sufyaankhan

### Tasks
- [ ] https://github.com/Agoric/dapp-agoric-basics/pull/8
- [ ] https://github.com/Agoric/dapp-agoric-basics/pull/10
- [ ] https://github.com/Agoric/dapp-agoric-basics/pull/11
- [ ] https://github.com/Agoric/dapp-agoric-basics/pull/21

[dckc]

more related issues:

• https://github.com/Agoric/documentation/issues/952
• https://github.com/Agoric/documentation/issues/951

[dckc]

@aj-agoric @sufyaankhan plans to work on this from my macbook this week are at-risk due to https://github.com/Agoric/agoric-3-proposals/issues/30#issuecomment-1964520858