internal/number: increase the max integer digits This allows int64s to convert without allocation. Change-Id: I6f1f95cb7cf72384ce9d420e05a077e378ebd7ad Reviewed-on: https://go-review.googlesource.com/46156 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>