v2.0.0
2.0.0: Transition to a stable collateral backing (USDT/USDC/etc). 1. Depricate `buy`/`sell` (restrict exchanging USN for NEAR). 2. Add deposit/withdraw mechanics to exchange USN for USDT/USDC/DAI collateral. 3. Commission: 0.01%. 4. `withdraw_stable_pool` for moving out a stable pool liquidity via DAO (up to 5%).