Exchange engine implementation

[featherenvy]

The exchange engine provides trading functionality and serves as a gateway for the trading engine.

Trading engine features:

• Support different exchanges using exchange adapters
• Rate limit support - exchange adapters will expose rate limits and exchange engine will stay within them
• Batch interface methods

trait RestExchangeAdapter {
  fn place_orders(&mut self, orders: &[OrderRequest]) -> Result<Box<[Order]>, PlaceOrdersError>;
  fn cancel_orders(&mut self, order_ids: &[OrderId]) -> Result<Box<[Order]>, CancelOrdersError>;
  fn fetch_balances(&self) -> Result<Box<[AssetBalance]>, FetchBalancesError>;
}

trait FixExchangeAdapter {
  fn connect(&mut self) -> Result<(), ConnectError>;
  fn place_order(&mut self, order: OrderRequest) -> Result<Order, PlaceOrderError>;
  fn cancel_order(&mut self, order_id: OrderId) -> Result<Order, CancelOrderError>;
}

trait WsExchangeAdapter {
  fn connect(&mut self) -> Result<(), ConnectError>;
}