encoding/hex: change lookup table from string to array

name            old time/op  new time/op  delta
Encode/256-4     431ns ± 2%   391ns ± 2%   -9.36%  (p=0.000 n=8+8)
Encode/1024-4   1.68µs ± 0%  1.51µs ± 0%   -9.91%  (p=0.001 n=7+7)
Encode/4096-4   6.68µs ± 0%  6.03µs ± 1%   -9.69%  (p=0.000 n=8+8)
Encode/16384-4  27.0µs ± 1%  24.0µs ± 0%  -11.03%  (p=0.000 n=8+7)

Change-Id: I6994e02f77797349c4e188377d84f97dffe98399
Reviewed-on: https://go-review.googlesource.com/27254


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>