[Zoe] OfferRules.offer is a confusing name

[katelynsills]

@Chris-Hibbert said:

There's a potential confusion between the offer's rules and the offer clause in the rules. ...we may want to consider renaming one of them.

To explain further, we call the entire structure "offerRules" and offerRules has a key "offer" that is what the person is offering.

Update (3/18):

In this morning's meeting, we decided to retain the name "offer" for the combination of the offerRules, payments, and invite as arguments to zoe.redeem.

We are changing offerRules to proposal.
We are changing offerRules.offer to give
want and exit in the offerRules (now proposal) remain the same.

In summary: A user makes an offer with Zoe. Their offer has a proposal with the following structure:

{
    give: { },
    want: { },
    exit: { },
}

This Issue should be closed in #685

[Chris-Hibbert]

To be even more explicit, offerRules (currently) have two keys: 'offer' and 'want'. We need a pair of terms, plus a term for the overall structure.

[katelynsills]

Closed by #685