encoding/hex: implement Decode with a lookup table

Implement hex decode using a 256 byte lookup table instead of branching logic.

In happy flow, uses 3x 64 byte (or 5x 32 byte) cache lines.

	name             old time/op    new time/op    delta
	Decode/256-64       223ns ± 3%     135ns ± 2%  -39.64%  (p=0.000 n=8+8)
	Decode/1024-64      872ns ± 2%     512ns ± 2%  -41.25%  (p=0.000 n=8+8)
	Decode/4096-64     3.43µs ± 1%    2.01µs ± 2%  -41.31%  (p=0.001 n=7+7)
	Decode/16384-64    13.9µs ± 1%     8.0µs ± 1%  -42.69%  (p=0.000 n=8+7)

	name             old speed      new speed      delta
	Decode/256-64    1.15GB/s ± 3%  1.90GB/s ± 2%  +65.66%  (p=0.000 n=8+8)
	Decode/1024-64   1.17GB/s ± 2%  2.00GB/s ± 2%  +70.22%  (p=0.000 n=8+8)
	Decode/4096-64   1.20GB/s ± 1%  2.04GB/s ± 2%  +70.39%  (p=0.001 n=7+7)
	Decode/16384-64  1.18GB/s ± 1%  2.06GB/s ± 1%  +74.49%  (p=0.000 n=8+7)

Also reduces amd64 object size by 766 bytes, despite the extra RODATA due to removal of `fromHexChar()` and duplicated inlined versions of it and simplification of `Decode()`.

Change-Id: I0988c7a30562ec154eff11db6e27954e0ce2b611
GitHub-Last-Rev: 64818018afc83ab07ec128a46aaea6a16f11400e
GitHub-Pull-Request: golang/go#51432
Reviewed-on: https://go-review.googlesource.com/c/go/+/390037


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>