math/fixed: add Mul methods.

Change-Id: Id6f9f5825527b311b5b1aa4ae0923c9551fa076b
Reviewed-on: https://go-review.googlesource.com/27413
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed