encoding/hex: pre-allocate Dump buffer

encoding/hex: pre-allocate Dump buffer

name          old time/op    new time/op    delta
Dump/256-4      7.76µs ± 2%    5.82µs ± 2%  -24.91%  (p=0.008 n=5+5)
Dump/1024-4     28.4µs ± 2%    22.6µs ± 3%  -20.58%  (p=0.008 n=5+5)
Dump/4096-4      112µs ± 2%      88µs ± 0%  -20.80%  (p=0.016 n=5+4)
Dump/16384-4     444µs ± 3%     361µs ± 7%  -18.73%  (p=0.008 n=5+5)

name          old alloc/op   new alloc/op   delta
Dump/256-4      4.00kB ± 0%    1.39kB ± 0%  -65.20%  (p=0.008 n=5+5)
Dump/1024-4     16.2kB ± 0%     5.5kB ± 0%  -66.04%  (p=0.008 n=5+5)
Dump/4096-4     63.9kB ± 0%    20.6kB ± 0%  -67.78%  (p=0.008 n=5+5)
Dump/16384-4     265kB ± 0%      82kB ± 0%  -69.00%  (p=0.008 n=5+5)

name          old allocs/op  new allocs/op  delta
Dump/256-4        7.00 ± 0%      3.00 ± 0%  -57.14%  (p=0.008 n=5+5)
Dump/1024-4       9.00 ± 0%      3.00 ± 0%  -66.67%  (p=0.008 n=5+5)
Dump/4096-4       11.0 ± 0%       3.0 ± 0%  -72.73%  (p=0.008 n=5+5)
Dump/16384-4      13.0 ± 0%       3.0 ± 0%  -76.92%  (p=0.008 n=5+5)

Change-Id: I0a0d6de315b979142b05e333880da8a5e52b12ef
Reviewed-on: https://go-review.googlesource.com/116495
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>