enable code-reuse for partial fill fraction calculations

[cbeck88]

In several places in the code around input rule validation, transaction building with SCIs, and soon mobilecoind, we will have code that performs fractional calculations with u64's by widening to u128's and then multiplying and dividing etc.

When I wrote this code Eran pointed out that we should avoid code duplication here, but I didn't see a good way at the time and I didn't do it.

Now I'm thinking, we should have like a utility class like U64Ratio or something that can capture a lot of this logic, and make it easier for it to work the same way everywhere.

I'm not sure I want to do this in the release/v4.1 branch, but I'd like to do it in master for 5.0 or something.

[cbeck88]

this is addressed in: #3214

and: #3224

with the U64Ratio class