Implemented via the underlying Int.SetUint64.
Added tests for Rat.SetInt64 and Rat.SetUint64.

Fixes #29579

Change-Id: I03faaffc93e36873b202b58ae72b139dea5c40f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/160682
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>