DEX: use Decimal for price, calculate it automatically

[Warchant]

Current make_order has this interface:

pub fn make_order(
	origin: OriginFor<T>,
	asset_id_1: u32,
	asset_id_2: u32,
	offered_amount: BalanceOf<T>,
	requested_amount: BalanceOf<T>,
	price: BalanceOf<T>,
	order_type: OrderType,
	expiration_block: BlockNumberFor<T>,
) -> DispatchResultWithPostInfo {

BalanceOf here is u128 - an integer.

A little bit further in this function this check is executed:

match order_type {
	OrderType::SELL => {
		ensure!(
			offered_amount * price == requested_amount,
			Error::<T>::PriceDoNotMatchOfferedRequestedAmount
		);
	}
	OrderType::BUY => {
		ensure!(
			offered_amount == requested_amount * price,
			Error::<T>::PriceDoNotMatchOfferedRequestedAmount
		);
	}
}

We should not do this check because offered_amount and requested_amount can be integers which represent assets with different precisions. We must convert them to a common precision before we can do multiplication or division.

Example:
I create assets A and B, and I want to SELL 5000A for 500B. With current design it is not possible, because:

offered_amount * price == requested_amount,
5000 * price == 500

price would have to be 0.1, but it has an integer type! So this trade is impossible.

The task contains these sub-tasks:

• # 334
• Price must not be passed as an argument from a user, because a user provides offered_amount and requested_amount
• Price must be calculated automatically inside make_order - because user provided offered_amount and requested_amount.