Time#in_milliseconds preserves millisecond precision

`Time` stores the number of nanoseconds since the Epoch.
`Time#in_milliseconds` should return the number of milliseconds since the
Epoch with a millisecond accuracy.
Ultimately this shouldn't matter, because the difference is quite small and this is not likely to be a performance bottleneck for any real usage.

I guess we can just do `(self * 1000).to_i` – this should work correctly for both float-y and integer-y types?