time: add methods to convert duration to microseconds and milliseconds

The return values are integers, as opposed to floats, since the fractionals can be derived from multiplying t.Seconds().

Fixes #28564

Change-Id: I3796227e1f64ead39ff0aacfbdce912d952f2994
GitHub-Last-Rev: b843ab740bf5a8216478322533521d6243fe1cb1
GitHub-Pull-Request: golang/go#30819
Reviewed-on: https://go-review.googlesource.com/c/go/+/167387


Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>