Fix protocol vs. pool fee calculations

[rowgraus]

No description provided.

[dckc]

I'm inclined to add to the description:

The rules that drive the design include

• When the user names an input (or output) price, they shouldn't pay more
  (or receive less) than they said.
• The pool fee is charged against the computed side of the price.
• The protocol fee is always charged in RUN.
• The fees should be calculated based on the pool balances before a transaction.
• Computations are rounded in favor of the pool.

[dckc]

This part seems to belong as well:

The user can specify a maximum amount they want to pay or a minimum amount they
want to receive. Unlike Uniswap, this approach will charge less than the user
offered or pay more than they asked for when appropriate. By analogy, if a user
is willing to pay up to $20 when the price of soda is $3 per bottle, it would
give 6 bottles and only charge $18. Uniswap doesn't adjust the provided price,
so it charges $20. This matters whenever the values of the smallest unit of the
currencies are significantly different, which is common in DeFi. (We refer to
these as "improved" prices.)

-- https://github.com/Agoric/agoric-sdk/blob/master/packages/zoe/src/contracts/constantProduct/README.md

cc @katelynsills

[dckc]

Around Oct 1, I wrote in #3791:

I suspect there is a AMM design document that I missed where I could confirm that @rowgraus , @btulloh , @dtribble are content that we don't require finer resolution than a basis point for fee ratios.

but I learned that I didn't miss such a document after all.

cc @katelynsills

[Chris-Hibbert]

The only issue mentioned above that hasn't been addressed is whether we require a finer resolution for fee ratios than 1 BP.

I think we've "fixed protocol vs. pool fee calculations".