LibWeb: Refactor XHR (almost) exactly to the spec

This makes XHR now rely on Fetch, which allows it to correct send
Origin and Referer headers, CORS-preflight and filtering and many other
goodies.

The main thing that's missing is Streams, which means we can't properly
produce progress events or switch to the Loading ready state.

This also doesn't implement the Document responseType just yet.

Userland/Libraries/LibWeb/Infra/ByteSequences.cpp

@@ -25,4 +25,33 @@ void byte_uppercase(ByteBuffer& bytes)
         bytes[i] = to_ascii_uppercase(bytes[i]);
 }
 
+// https://infra.spec.whatwg.org/#byte-sequence-starts-with
+bool is_prefix_of(ReadonlyBytes potential_prefix, ReadonlyBytes input)
+{
+    // "input starts with potentialPrefix" can be used as a synonym for "potentialPrefix is a prefix of input".
+    return input.starts_with(potential_prefix);
+}
+
+// https://infra.spec.whatwg.org/#byte-less-than
+bool is_byte_less_than(ReadonlyBytes a, ReadonlyBytes b)
+{
+    // 1. If b is a prefix of a, then return false.
+    if (is_prefix_of(b, a))
+        return false;
+
+    // 2. If a is a prefix of b, then return true.
+    if (is_prefix_of(a, b))
+        return true;
+
+    // 3. Let n be the smallest index such that the nth byte of a is different from the nth byte of b.
+    //    (There has to be such an index, since neither byte sequence is a prefix of the other.)
+    // 4. If the nth byte of a is less than the nth byte of b, then return true.
+    // 5. Return false.
+    for (size_t i = 0; i < a.size(); ++i) {
+        if (a[i] != b[i])
+            return a[i] < b[i];
+    }
+    VERIFY_NOT_REACHED();
+}
+
 }