internal/number: add AppendDigit the other ones may go eventually. Change-Id: I6c34f6972c1a827eb4dc193c5be26bb686681e7f Reviewed-on: https://go-review.googlesource.com/45492 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>