Add nanosecond-precision clock

[erikc5000]

The current clock implementation is limited to millisecond precision, which is the best you can get with Android or Java 8 anyway. But on Java 9 and iOS/macOS, microsecond resolution is available.

Millisecond resolution may be fine for most and should be a bit faster -- certainly, it'll result in fewer allocations when getting the current time. The default should probably be nanosecond precision where available, but providing the option to use a faster millisecond precision clock would be the best of both worlds.

To support nanosecond precision time on Java 9, we'd want to introduce a PlatformInstant expect class that is implemented by Java's Instant class and wrap Java's Clock. This should keep additional allocations to a minimum. Since we'd be using java.time classes under the hood, this implementation won't work on Android though, so it'll require a new target or project rearrangement of some sort.