Update timestamp to support unlimited precision

Currently Ion Go timestamps supports [9 fractional precision](https://github.com/amzn/ion-go/releases/tag/v0.1.0-beta).

We should update timestamps to support unlimited precision to conform to the [spec](https://amzn.github.io/ion-docs/docs/spec.html#timestamp):

> Fractional seconds are allowed, with at least one digit of precision and an unlimited maximum.