unicode/utf8: make FullRune inlinable

unicode/utf8: make FullRune inlinable

This has same readability and allows to inline FullRune for massive
performance gain:

FullASCIIRune-6                      4.36ns ± 0%  1.25ns ± 0%  -71.33%  (p=0.000 n=8+10)
FullJapaneseRune-6                   4.70ns ± 0%  1.42ns ± 1%  -69.68%  (p=0.000 n=9+10)

Change-Id: I95edd6292417a28aac244e40afb713596a087d93
Reviewed-on: https://go-review.googlesource.com/63332
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>