Reduces duplication and centralizes documentation.
Moves all uses of FmtUnsigned and tconv inside fmt.go.

Passes toolstash -cmp.

Change-Id: If6d906e7e839de05f36423523a3a1d596e29807d
Reviewed-on: https://go-review.googlesource.com/38141
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>