refactor: move util::Xor to Obfuscation().Xor

This is meant to focus the usages to narrow the scope of the obfuscation optimization.

`Obfuscation::Xor` is mostly a move.

Co-authored-by: maflcko <6399679+maflcko@users.noreply.github.com>

src/util/obfuscation.h

@@ -6,11 +6,29 @@
 #define BITCOIN_UTIL_OBFUSCATION_H
 
 #include <cstdint>
+#include <span.h>
 
 class Obfuscation
 {
 public:
     static constexpr size_t KEY_SIZE{sizeof(uint64_t)};
+
+    void Xor(std::span<std::byte> write, std::span<const std::byte> key, size_t key_offset = 0)
+    {
+        assert(key.size() == KEY_SIZE);
+        key_offset %= KEY_SIZE;
+
+        for (size_t i = 0, j = key_offset; i != write.size(); i++) {
+            write[i] ^= key[j++];
+
+            // This potentially acts on very many bytes of data, so it's
+            // important that we calculate `j`, i.e. the `key` index in this
+            // way instead of doing a %, which would effectively be a division
+            // for each byte Xor'd -- much slower than need be.
+            if (j == KEY_SIZE)
+                j = 0;
+        }
+    }
 };
 
 #endif // BITCOIN_UTIL_OBFUSCATION_H