servicefee: use an ordered map to make evaluation of expressions more deterministic

[zikani03]

The servicefee.FeeExpressions type is an alias of a map[string]float64 which means that it inherits the built in map's behavior which is especially apparent when iterating over the expressions to calculate a fee.

From: https://go.dev/ref/spec#For_range

The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next....

The effect is such that if you specify the fees as below, the service fee for the amount 10,000 for example can be either 500.0 OR 5.0 - we need this to be more deterministic, and have the rules evaluated in the order they are specified

	serviceFees := servicefee.NewFixedFee(servicefee.FeeExpressions{
		"x <= 5000":               200.00,
		"x >= 5000 && x <= 10000": 500.00,
		"x > 1000":                5.0,
	})

Try on Go Play Ground: https://go.dev/play/p/4FiMV6jd6cI

[Horac3]

Can you add more context to the description?

[zikani03]

@Horac3 i have updated the description. It would also be interesting to look into mutation testing for this, but that doesn't have to be part of the PR for this issue.

[Horac3]

I should be able to push a PR anytime soon