bytes: encode size of rune read by ReadRune into lastRead to speed up UnreadRune

bytes: encode size of rune read by ReadRune into lastRead to speed up UnreadRune

In ReadRune store the size of the rune that was read into lastRead
to avoid the need to call DecodeRuneLast in UnreadRune.

fmt:
name        old time/op  new time/op  delta
ScanInts-4   481µs ± 4%   458µs ± 3%  -4.64%  (p=0.000 n=20+20)

Change-Id: I500848e663a975f426402a4b3d27a541e5cac06c
Reviewed-on: https://go-review.googlesource.com/28817
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>