RFC: Higher level interface for applying promotions

[jhawthorn]

Our current promotion interface often doesn't allow the desired behaviour.

We want all promotions to calculate a promotion amount at a line-item level, but often stores want to emulate a per-order discount. This results in calculators like DistributedAmount which attempt to do this, but are awkward and involve error prone recalculations involving the whole order per-line item.

I believe we should take the lessons learned addressing similar issues in taxation #1892, and try to apply them to promotions.