Support percent values

[totemcaf]

Money is usually used in context when a percent is computed on some money value, for example to compute an interest amount.

A Percent type could be added as a companion abstraction for money.

// Percent is a 3 decimal percent value. Internally it is stored as an int64 with a 3 digits scale
//   interestRate := percent.FromStr("3.5")
// It represents a 3.5% (a 0.03500) factor, and stored as 3500
type Percent int64

const Scale = 1000
const ScaledPercentToRate = 100 * Scale

func (p Percent) By(amount fpmoney.Amount) (computed fpmoney.Amount, remainder fpmoney.Amount) {
    return amount.Mul(int(p)).Div(ScaledPercentToRate)
}

Additional methods will be provided.

What do you think?

I can provide a PR.

[nikolaydubina]

Hi, yes, that is useful too. But I am not sure how much precision is necessary. I could imagine people want to have more than 1 fraction decimal for percentages.

Let's keep fpamount to deal only with {"amount" ... "currency"} pairs.

Do recommend for you to create new module / repo for percentages though if you are interested! :D