AMM: fix computation of output amount for stable pools

[peterslany]

Computation of output amount for stable pools is returning incorrect results when pool is out of balance.

https://discord.com/channels/745259537707040778/1072820080905302126/1072820084520779816

Testnet with issue at kintnet-api.interlay.io

[gregdhill]

I think this is to be expected, Curve v1 only supports stable (pegged) assets but on testnet the LKSM-VKSM-SKSM pool was not balanced - LKSM is worth significantly less than VKSM and SKSM. The solution is to rebase the token on the parachain, see: interlay/interbtc#935