Stable AMM doesn't support price imbalance

[gregdhill]

Describe the bug
When trading against a base pool with three assets where the input currency is worth significantly more, the output amount should be less.

To Reproduce
Steps to reproduce the behavior:

1. Create a base pool with assets A, B, and C
2. Add liquidity 1e12, 1e11 and 1e11 respectively
3. Trade A to B
4. Output amount is almost the same as input

Expected behavior
Output amount should be less since the price of the input currency is less

Additional context
See: #920

[gregdhill]

Seems this is expected behavior, Curve v1 only supports pegged assets. In the case of LKSM Acala converts this value to KSM here: https://github.com/AcalaNetwork/Acala/blob/01f19ae1bfb0afff62a0b938c8e2d02cf5d831c5/runtime/karura/src/lib.rs#L1594-L1630

[gregdhill]

Created #1036