Margin account: health

The dango-lending contract stores a collateral power for each collateral asset, and a Market for each borrowable asset:

#![allow(unused)]
fn main() {
const COLLATERAL_POWERS: Item<BTreeMap<Denom, Udec128>> = Item::new("collateral_power");

const MARKETS: Map<&Denom, Market> = Map::new("market");
}

• An asset may be a collateral asset but not a borrowable asset, e.g. wstETH, stATOM, LP tokens. But typically all borrowable assets are also collateral assets, such that when a margin account borrows an asset, this asset counts both as collateral and debt.
• Collateral powers are to be bounded in the range [0, 1). An asset with lower volatility and more abundant liquidity gets bigger collateral power, vise versa.
• We may store all collateral powers in a single Item<BTreeMap<Denom, Udec128>> if we don't expect to support too many collateral assets.

Suppose:

• a margin account has collateral assets $A_1,A_2,\dots ,A_n$ and debts $B_1,B_2,\dots ,B_m$
• the price of an asset $X$ is $P_X$
• the collateral power of an asset $X$ is $C_X$

The account's utilization is:

$$U=\frac{{\sum }_{j=1}^m{P}_{B_j}{B}_j}{{\sum }_{i=1}^n{C}_{A_i}{P}_{A_i}{A}_i}$$

In the backrun function, the margin account asserts $U\le 1$. If not true, it throws an error to revert the transaction.

The frontend should additionally have a max_ltv, somewhat smaller than 1, such as 95%. It should warn or prevent users from doing anything that results in their utilization going bigger than this, such that their account isn't instantly liquidated.