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>
2 files changed