Manually convert hex escape sequence to rune instead of calling
strconv.ParseUint.

This inlines the unhex func from docs (and many other packages).

name              old time/op    new time/op    delta
UnicodeDecoder-4     468ns ± 1%     402ns ± 1%  -14.26%  (p=0.000
n=10+10)

name              old speed      new speed      delta
UnicodeDecoder-4  29.9MB/s ± 1%  34.8MB/s ± 1%  +16.59%  (p=0.000
n=10+10)

name              old alloc/op   new alloc/op   delta
UnicodeDecoder-4     44.0B ± 0%     36.0B ± 0%  -18.18%  (p=0.000
n=10+10)

name              old allocs/op  new allocs/op  delta
UnicodeDecoder-4      4.00 ± 0%      2.00 ± 0%  -50.00%  (p=0.000
n=10+10)

Fixes #20567

Change-Id: If350978d5bb98ff517485752184d02249f5d1f3a
Reviewed-on: https://go-review.googlesource.com/44738
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>