refactor: simplify reducing solution

lib/bill-math.js

@@ -129,24 +129,13 @@ const getSolution = (units, amount) => {
 
 const solutionToOriginalUnits = (solution, units) => {
   const billsToAssign = (count, left) => _.clamp(0, count)(_.isNaN(left) || _.isNil(left) ? 0 : left)
-
-  const billsLeft = _.flow(
-    _.map(([denomination, provisioned]) => [BN(denomination), provisioned]),
-    _.reduce((acc, value) => {
-        acc[value[0]] = (acc[value[0]] || BN(0)).plus(value[1])
-        return acc
-      },
-      {}
-    )
-  )(solution)
-
-  return _.map(
+  const billsLeft = Object.fromEntries(solution)
+  return units.map(
     ({ count, name, denomination }) => {
       const provisioned = billsToAssign(count, billsLeft[denomination])
       billsLeft[denomination] -= provisioned
       return { name, denomination, provisioned }
-    },
-    units
+    }
   )
 }