pull up .times operation to Money class

Tasklist:
- $5 + 10CHF = $10 if rate is 2:1 🎯
- $5 * 2 = $10 ✅
- Make "amount" private ✅
- Dollar side-effects? ✅
- Money rounding?
- equals() ✅
- Equal null
- Equal object
- 5 CHF * 2 = 10 CHF ✅
- Dollar/Franc duplication
- Common `.equals` ✅
- Common `.times` 👈🏼
- Compare Francs with Dollars ✅
- Currency? ✅
- Merge multiplication tests?

src/money/index.ts

@@ -25,24 +25,16 @@ class Money {
   }
 
   times(multiplier: number): Money {
-    return new Money(0, 'USD');
+    return new Money(this.amount * multiplier, this.currency);
   }
 
   get currency(): string {
     return this._currency;
   }
 }
 
-export class Dollar extends Money {
-  times(multiplier: number): Money {
-    return new Money(this.amount * multiplier, this.currency);
-  }
-}
+export class Dollar extends Money {}
 
-export class Franc extends Money {
-  times(multiplier: number): Money {
-    return new Money(this.amount * multiplier, this.currency);
-  }
-}
+export class Franc extends Money {}
 
 export default Money;