RatedSwapPool for stNEAR/wNEAR pool

[vitalii427]

Generic RatedSwapPool and implementation for stNEAR/wNEAR pool

Based on StableSwapPool source code

Major changes:

• added rates to swap & add/predict/remove_liquidity calculations
• math upgraded to 24 decimals using U384
• generic rates acquisition from another smart contract via cross-contract call with caching
• minimum boilerplate for rates acquisition implementation
• includes sample implementation of rates acquisition for stNEAR

New external methods:

• add_rated_swap_pool
• add_rated_liquidity
• rated_swap_ramp_amp
• rated_swap_stop_ramp_amp
• get_rated_pool [view]
• get_rated_return [view]
• predict_add_rated_liquidity [view]
• predict_remove_rated_liquidity_by_tokens [view]
• update_pool_rates
• update_pool_rates_callback [callback]

Add liquidity flow:

• call get_rated_pool & check rates are actual
• call predict_add_rated_liquidity with actual rates
• batch-call [update_pool_rates, add_rated_liquidity]

Remove liquidity by tokens flow:

• call get_rated_pool & check rates are actual
• call predict_remove_rated_liquidity_by_tokens with actual rates
• batch-call [update_pool_rates, remove_liquidity_by_tokens]

Remove liquidity by shares flow:

• same as StableSwapPool

Swap flow:

• call get_rated_pool & check rates are actual
• call get_rated_return with actual rates
• batch-call [update_pool_rates, swap]

Rates acquisition implementation:

• implement instance of Rates enum with RatesTrait

pub trait RatesTrait {
    /// Check that cached rates are actual
    fn are_actual(&self) -> bool;
    /// Get cached rates vector
    fn get(&self) -> &Vec<Balance>;
    /// Update cached rates
    ///  if cached rates are actual returns true
    ///  else returns cross-contract call promise
    fn update(&self) -> PromiseOrValue<bool>;
    /// Update callback
    ///  receives JSON encoded cross-contract call result
    ///  and updates cached rates
    fn update_callback(&mut self, cross_call_result: &Vec<u8>) -> bool;
}

Other:

• unit tests added/fixed