Implement an option for millisecond time.

[player-03]

For use cases that require integers, as in #26.

Defines a new echoes.Time type that is fully backwards-compatible. For anyone who doesn't need integer time, you can continue using Float as before, or swap over to echoes.Time, or a mix of both.

If you need integer time, define echoes_millisecond_time. Then, update your code to use echoes.Time instead of Float, and millisecond values instead of seconds.