This PR adds savings calculations for swaps, in the swaps controller.

The function previously named _findTopQuoteAggId has been renamed _findTopQuoteAndCalculateSavings. This function already made the calculations to find the best quote by overall ETH value.

With very little added work, we now also calculate the overall swap fees in ETH, which include gas costs and all exchange fees except our own, which proportionally identical across all quotes and already subtracted by the time the quotes are received.

Finally, we calculate the savings for the best quote. In the UI, we will display these savings along with our fee. We calculate savings relative to the median fee and the median trade value for all quotes, compared to that of the best quote.

Here are the equations in pseudocode, assuming all units converted to ETH:

total_eth_cost = swap_tx_gas_cost + approval_tx_gas_cost + swap_tx_eth_sent

We say total_eth_cost, because it does not include the value of the source token unless the source token is ETH.

swap_tx_eth_sent includes third-party fees and, if the source token is ETH, the amount of ETH swapped.

trade_fee = total_eth_cost - (source_token_is_eth ? amount_of_eth_swapped : 0)

trade_value = destination_token_eth_value - total_eth_cost

For each quote, we store trade_fee and trade_value for use in subsequent calculations.

We want the trade value to be as high as possible, so we subtract the median from the best value.

performance_savings = best_quote_trade_value - median(all_trade_values)

We want the trade fees to be as low as possible, so we subtract the fees of the best quote from the median fee of all quotes.

fee_savings = median(all_trade_fees) - best_quote_trade_fee

Note that either component of total_savings can be negative, and we often observe one negative and one positive in practice.

total_savings = performance_savings + fee_savings