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>

1 files changed, 1 insertions, 0 deletions

diff --git a/api/next/53021.txt b/api/next/53021.txt
new file mode 100644
index 0000000000..3adb9b1198
--- /dev/null
+++ b/api/next/53021.txt
@@ -0,0 +1 @@
+pkg crypto/subtle, func XORBytes([]uint8, []uint8, []uint8) int #53021