JSON support

[jessealama]

Should Amount have a toJSON method? If so, perhaps the method could return something like

{ 
  "value": "123.456"
  "unit": "kilogram"
}

The value in unit could be undefined if the Amount doesn't have a unit.

Do we want to include the significant digits and/or fraction digits properties, too?

This issue touches on #35. If we round on the way in, then the string contained in the value property is precisely the value, so including significant digits and fraction digits is arguably unnecessary. However, if we round only on the way out (only), conversion to JSON might be like toNumber, toString in that toJSON would not just emit a JSON object, but also do rounding beforehand. Altneratively, one could include significant digits and fraction digits in the JSON:

let a = new Amount(123.456);
a.with({ fractionDigits: 2 }.toJSON();
// { "value": "123.46", "unit": undefined }

// or:

// { "value": "123.456", "fractionDigits": 2, "unit": undefined }

[jessealama]

(Looking at the hoops I jumped through in the original text, in thinking through what toJSON() should do if we opt for rounding only on the way out, I think the mess could be considered an argument against rounding only on the way out.)

[jessealama]

If we have toJSON(), perhaps we should accept the same JSON we emit in the constructor? Like this:

let a = new Amount(1.2345);
let b = new Amount(a.toJSON()); // probably shouldn't throw

[eemeli]

Two initial thoughts:

1. There's no need to express the value as anything other than a JSON number. We can do that with JSON.rawJSON().
2. Given that many use cases are likely to be working with unitless Amounts, the object wrapper doesn't always make sense.

[jessealama]

This issue also touches on #50 .