How to get the literal value of a number?

For instance, for json content

```json
{"value": 1.23456789234567892345678923456789234567892345678923456789}
```

I'd like to get a string-like `1.23456789234567892345678923456789234567892345678923456789` from parsed `value`.

This is useful not even for arbitrary precision floats, but for decimal floats. Number `0.1` has no accurate representation with IEEE754 binary float. Some users may want to pass this value as-is to their decimal float library, so they need the literal value.