encoding/hex: change lookup table back to string

encoding/hex: change lookup table back to string

CL 27254 changed hextable to a byte array for performance.
CL 28219 fixed the compiler so that that is no longer necessary.
As Kirill notes in #15808, a string is preferable
as the linker can easily de-dup it.
So go back. No performance changes.

Change-Id: Ibef7d21d0f2507968a0606602c5dd57ed4a85b1b
Reviewed-on: https://go-review.googlesource.com/40970
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>