Add must_pay_many

[shapeshed]

This requires that two and only two of the specified denoms are sent

[hashedone]

I don't think it is the best approach. must_pay is a convinence functinon for the super common case of one coin needed, so we want to provide the nice API for this case. I understand the need of accepting more coins for some cases, but I feel, that going with must_pay_two_coins we should also add must_pay_three_coins, must_pay_four_coins and so on - at least until clippy complains for too many args.
If you want to provide the better API for more than one coin, I suggest the function like must_pay_many(&[&str]) - with this generic signature you will support arbitrarly amount of tokens with fairly simplified API.

[shapeshed]

Ok fair point, I will have a think about reworking the api

[shapeshed]

Function signature updated to

pub fn must_pay_many(
    info: &MessageInfo,
    required_denoms: &[&str],
) -> Result<Vec<Coin>, PaymentError> {

Also updated to use a Hashmap and check for duplicates

[shapeshed]

Are you consider merging this?