Currency::minor_units can be removed

[alopatindev]

(Taking into account that Currency is Copy, so we put all its fields on stack) Currency::minor_units seems to be redundant, because it can be computed from something like dec!(10).powd(-Currency::exponent)

[davestearns]

I might be missing something, but I don't see any place in the code where the minor_units field is even used.

https://github.com/search?q=repo%3Avarunsrin%2Frusty_money+minor_units&type=code

It's defined for the currencies, but I don't see any code that actually uses it. I only see uses of exponent. So yeah, I think this field could just be removed, right? Or am I missing something?

Nice job on this library!