feat: change fee model to one that cannot be gamed

[fplord]

2 parameters maxLiquidityRemaining zeroLiquidityRemaining
if your swap leaves max liquidity remaining in the pool, you pay maxLiquidityRemaining% fees. If your swap leaves no liquidity remaining in the pool, you pay zeroLiquidityRemaining% fees

current fee model incentivises splitting up into smaller swaps so as to not incur higher fees: see the difference in fees paid (red rectangles' area) when you split unstake into two swaps versus doing it in one big swap

suggest changing this to the following:

• let the amount requested be amountRequested
• calculate feeAtCurrentLiquidity (linear interpolation between max and zero)
• calculate feeAtCompletedLiquidity (fee that will be charged after this swap, again, linear interpolation)

total fee charged as follows:

amountRequested * feeAtCurrentLiquidity + 1/2 * amountRequested * (feeAtCompletedLiquidity - feeAtCurrentLiquidity)

Compared to the current fee model, this fee model means that you cannot get a lower fee by splitting up your swap into smaller swaps

[fplord]

low priority but just something to keep in mind

[billythedummy]

amountRequested * feeAtCurrentLiquidity + 1/2 * amountRequested * (feeAtCompletedLiquidity - feeAtCurrentLiquidity)
= amountRequested * (feeAtCompletedLiquidity + feeAtCurrentLiquidity) / 2

(feeAtCompletedLiquidity + feeAtCurrentLiquidity) / 2 is average of the 2 fees