Pool: Replace belief_price with minimum expected amount

[ueco-jb]

This change set replaces the belief_price argument with the amount expected to receive in return after the swap.
Rationale: It's easier and more straightforward both implementation-wise and for the user to specify those amounts directly.

[gangov]

I had to stop by and check this. One small discrepancy between pool and pool_stable is the compute_swap functionality.

In pool we have it like:

pub fn compute_swap(
    offer_pool: i128,
    ask_pool: i128,
    offer_amount: i128,
    commission_rate: Decimal,
    referral_fee: i64,
) -> ComputeSwap {

while in pool_stable:

pub fn compute_swap(
    env: &Env,
    offer_pool: i128,
    ask_pool: i128,
    offer_amount: i128,
    commission_rate: Decimal,
) -> (i128, i128, i128) {

ComputeSwap has an extra field referral_fee_amount

I have knowingly left that as is, because I think it's okay and better to readability

[ueco-jb]

LGTM