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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,ImJasonH/go,vsekhar/elastic-go,golang/go,Cofyc/go,CAFxX/go,nawawi/go,CAFxX/go,josharian/go,cloudflare/go,derekmarcotte/go,polyverse-security/go,derekmarcotte/go,codestation/go,vsekhar/elastic-go,vsekhar/elastic-go,derekmarcotte/go,polyverse-security/go,cloudflare/go,golang/go,nawawi/go,Cofyc/go,codestation/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,polyverse-security/go,ImJasonH/go,ImJasonH/go,akutz/go,cloudflare/go,Cofyc/go,ImJasonH/go,nawawi/go,polyverse-security/go,polyverse-security/go,derekmarcotte/go,Cofyc/go,josharian/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,codestation/go,derekmarcotte/go,ImJasonH/go,cloudflare/go,josharian/go,akutz/go,Cofyc/go,golang/go,cloudflare/go,derekmarcotte/go,ImJasonH/go,ImJasonH/go,akutz/go,derekmarcotte/go,CAFxX/go,josharian/go,codestation/go,josharian/go,polyverse-security/go,Cofyc/go,josharian/go,christopher-henderson/Go,CAFxX/go,codestation/go,akutz/go,CAFxX/go,ImJasonH/go,golang/go,josharian/go,CAFxX/go,akutz/go,CAFxX/go,vsekhar/elastic-go,nawawi/go,derekmarcotte/go,codestation/go,josharian/go,golang/go,golang/go,Cofyc/go,nawawi/go,golang/go,akutz/go,vsekhar/elastic-go,codestation/go,nawawi/go,polyverse-security/go,Cofyc/go,akutz/go,nawawi/go,codestation/go,akutz/go,vsekhar/elastic-go