[webpayments] Expression of monetary amounts (#40)

The paymentRequest API currently suggests that we express monetary amounts using a mantissa and exponent: 

http://wicg.github.io/paymentrequest/specs/paymentrequest.html#currencyamount
```javascript
"amount": { "currencyCode": "USD", "mantissa": 5500, "exponent": 2 }, // US$55.00
```

The Web Payments CG Browser API suggests that we use strings for the amounts with a currency code:

http://wicg.github.io/web-payments-browser-api/#processing-a-payment-request
```javascript
transfer: {
  amount: '4.35',
  currency: 'USD'
},
```

How should monetary amounts be expressed in the API?

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webpayments/issues/40