RangeIonTimestamp ignores precision when comparing timestamps

[zslayton]

When constructing a range of IonTimestamps, we currently convert the input IonTimestamp values to a BigDecimal number of milliseconds since the epoch using IonTimestamp#getDecimalMillis:

https://github.com/amzn/ion-schema-kotlin/blob/cde794e69206504227c8526580c28a56dedf18db/src/com/amazon/ionschema/internal/util/RangeIonTimestamp.kt#L39-L47

This means that we erroneously allow before/after comparisons of timestamps with mixed precision. For example, we consider 2007-06-01T12:46 to be after 2007-06-01T, which we cannot definitively conclude.

We will also ignore precisions greater than milliseconds, considering 2007-06-01T12:46:00.000111 and 2007-06-01T12:46:00.000444 to be equivalent.