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/28817Reviewed-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>