[Zoe] Add roles

[katelynsills]

This is a WIP PR to add "roles" to Zoe.
Closes #97
Closes #392
Closes #469
Closes #564
Closes #677

Tests that don't yet pass:

Kate responsibility:

• zoe - autoswap - valid inputs - with SES
• autoSwap with valid offers
• autoSwap - test fee

Chris responsibility:

• zoe - simpleExchange - valid inputs - with SES
• zoe - simpleExchange - state Update - with SES
• myFirstDapp with valid offers
• myFirstDapp with multiple sell offers
• myFirstDapp showPayoutRules
• simpleExchange with valid offers
• simpleExchange with multiple sell offers
• simpleExchange showPayoutRules

Still to do:

• fix autoswap
• fix simpleExchange
• fix myFirstDapp
• Remove old code and helpers
• Assess whether old payoutRules structure is still helpful at all
• Look over and clean up
• Close this Draft and reopen a new PR for the Zoe release branch

[katelynsills]

This PR is ready for an initial design review. I would recommend starting with the coveredCall test (test.only: zoe - coveredCall) and looking at the changes from the user's perspective, then looking at Zoe.js and state.js.

I'll continue updating the tests while the review is happening in parallel.

[Chris-Hibbert]

I will look at zoe.js tomorrow. This is as far as I got today.

[erights]

Review so far. I like where this is going --- the changes look much better than the original. This is a much better API.

I'm puzzled by enough things that I'll wait for some responses before resuming the review.

[katelynsills]

@Chris-Hibbert I added TODOs to the PR description. I assigned you to the SimpleExchange and MyFirstDapp changes. Let me know if that works!

[Chris-Hibbert]

Yep. I'll work on those. I've pulled down your recent changes to the contracts and will continue from there.

[Chris-Hibbert]

checkIfOfferRules looks helpful. It's a little surprising that it didn't reduce the amount of code, but I think it does clarify the intent. I'll look into using it for simpleExchange.

[katelynsills]

Closed in favor of #685