[Zoe] Allow users of smart contracts to make their payout rules for any subset of assays

[katelynsills]

For example, a user trying to swap moola for simoleans in autoswap would not need to mention the liquidity assay:

 const bobMoolaForSimOfferRules = harden({
      payoutRules: [
        {
          kind: 'wantAtLeast',
          units: simoleans(1),
        },
        {
          kind: 'offerAtMost',
          units: moola(3),
        },
      ],
      exitRule: {
        kind: 'onDemand',
      },
    });

Related issue: #97