crypto/subtle: add XORBytes

crypto/subtle: add XORBytes

Export cipher.xorBytes as subtle.XORBytes, for proposal #53021,
to provide fast XOR to cryptography libraries outside crypto/cipher.

Along with the move, implement the alignment check TODO
in xor_generic.go, so that systems with neither unaligned
accesses nor custom assembly can still XOR a word at a time
in word-based algorithms like GCM. This removes the need
for the separate cipher.xorWords.

Fixes #53021.

Change-Id: I58f80a922f1cff671b5ebc6168eb046e702b5a4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/421435
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>