[go1.17] internal/subtle xor 简化代码，去除重复

[emmansun]

amd64/arm64使用指针：

//go:noescape
func xorBytes(dst, a, b *byte, n int)

而generic实现使用切片：

// fastXORBytes xors in bulk. It only works on architectures that
// support unaligned read/writes.
// n needs to be smaller or equal than the length of a and b.
func fastXORBytes(dst, a, b []byte, n int) {

golang 1.17提供了unsafe.Slice方法，可用用来简化代码：

func xorBytes(dstb, xb, yb *byte, n int) {
	// xorBytes assembly is written using pointers and n. Back to slices.
	dst := unsafe.Slice(dstb, n)
	x := unsafe.Slice(xb, n)
	y := unsafe.Slice(yb, n)
...
}