Use nanosecondsSinceEpoch in `SpanEvent`

[slashmo]

In #114 we added nanosecondsSinceEpoch to the TracerInstant. SpanEvent, however, still uses millisecondsSinceEpoch.
We should also base SpanEvent on nanoseconds, exposing the same convenience millisecondsSinceEpoch accessor we do with TracerInstant.