Xyk bonding curve

[0xacedia]

Adds support for an x*y=k style bonding curve.

After discussion:

The x and y are based on virtual amounts not actual amounts.
This allows inventory to be decoupled from price/slippage.

For example, Alice may set her x to be 30 eth and her y to be 3 nfts.
But she may then only deposit 10 eth and 1 nft.
That means that the curve will move along as if there was 30 eth and 3 nfts of liquidity.
Similarly she may also specify a different ratio. x = 20 eth, y = 3 nfts (price = 20 / 3 = 6.67).
And then deposit 10 eth and 1 nft (price = 10 / 1 = 10.0).
This means that the curve will move along as if it had 20 eth and 3 nfts of liquidity.

Of course, if the virtual reserves don't match up with the actual reserves then this means that it's possible for a pool to run out of liquidity if it gets sufficiently far along the curve. However this is no different than how the linear and exponential curves behave. The key thing is the way that the curve is shaped: x*y=k.

[0xacedia]

Had to make a few changes to the test-cases to make them work with the xyk curve. Mainly just removing some of the fixed parameters in RouterRobustSwap, RouterSinglePool and RouterSinglePoolWithAssetRecipient.