[Zoe] Autoswap: Allow users to create new liquidity pools in the same contract

[katelynsills]

Our current implementation of autoswap assumes two kinds of tokens and one liquidity pool. We would like to allow users to create new liquidity pools for a new pair of tokens if the particular liquidity pool does not already exist.

The end goal is that a user would be able to trade x for z by using two liquidity pools: (x, y) and (y, z).

Assumptions:

• Smart contracts will be instantiated with terms, which include an array of assays to start with.
• Contracts can reject these user defined terms and can reject offers.
• Contracts can asynchronously add a new assay to the end of the canonical array of assays. This is not synchronous because the new assay may be remote so we must wait for the remote response.
• Contracts can have methods that allow external users to add assays to the contract. (Compare to the Uniswap factory.)

The following changes are necessary to make this possible:

• Allow users of smart contracts to make their offer rules with any order of assays [Zoe] Allow users of smart contracts to make their offer rules with any order of assays #97
• Allow users of smart contracts to make their payout rules for any subset of assays [Zoe] Allow users of smart contracts to make their payout rules for any subset of assays #392
• Allow smart contracts to add assays to their canonical list. [Zoe] Part 6 of Switching to Zoe Invites (Autoswap) #380
• Allow smart contracts to reallocate units for a subset of the canonical list of assays. (It's not clear whether this should be a helper filling out the unitMatrix or whether it should be built in.) [Zoe] Allow smart contracts to reallocate units for a subset of the canonical list of assays  #393

[katelynsills]

Closed by #861