Add Multiples for Zoe offers

[Chris-Hibbert]

What is the Problem Being Solved?

@erights wrote a draft implementation of multiples, which resolves our old desire for divisible contracts. It needs to be carried across the goal line.

Description of the Design

remaining tasks:

• Make the PR be green on CI. Tests are currently failing, but we haven't investigated why.
• make it opt-in.
  • Current idea is to include an extra option to makeInvitation()
  • One alternative is to make it require an explicit opt-in by the contract, though there's no proposed mechanism
• review whether there are interactions between offers with and without multiples specified. (Hypothese: no, since unspecified=1).
• Verify that zoeHelpers don't need to be updated.
• update agoric.com/Documentation with at least a hint of this feature.
• update isOfferSafe jsDoc

Do we need a flag on Zoe to disable use until we decide it should be available? [Proposed answer: No global flag is necessary.]

Security Considerations

Offer safety is paramount.

Test Plan

• Add tests, including at least a trivial contract making use of the feature.
• Test for interactions between offers that specify multiples of atomic units that aren't multiples of each other.
  (I'm thinking of a simpleExchange between "up to 5 copies of three items", and "up to 7 copies of two items", and whether isOfferSafe is going to be confused when the contract propose to reallocate 10 items.)

[dckc]

Doing this in Mainnet 1.1 requires a Zoe upgrade, yes? I guess I'll mark this as blocked by it... or at least: by Zoe durability (#4383), which is the closest thing I see.