v2.1.0
2.1.0: Transition to a stable collateral backing (USDT). 1. Depricate `buy`/`sell` (restrict exchanging USN for NEAR). 2. Add deposit/withdraw mechanics to exchange USN for USDT collateral. 3. Commission: 0.05%. 4. `withdraw_stable_pool` for moving out a stable pool liquidity via DAO (up to 5%).