QRingBuffer: optimize and simplify QRingChunk::toByteArray() futher

- use sliced() instead of mid(), since we know we call sliced()
  in-contract

- replace the manual memmove() with calls to QBA::remove(), but first
  resize() to tailOffset, and only then remove(0, headOffset), so we
  only move Chunk::size() bytes, as the old code did

- don't bother updating the offset members, as we're restricted to
  rvalues now, and we only need to leave behind a partially-formed
  object

This code shows that we desperately need QBA::slice() and/or an rvalue
overload of QBA::sliced(), cf. QTBUG-99218. Add a comment to use these
functions when they become available.

Change-Id: I5d8c7363d443dff69338f6f6a7b4bff9957917ec
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

src/corelib/tools/qringbuffer.cpp

@@ -71,18 +71,13 @@ void QRingChunk::detach()
 
 QByteArray QRingChunk::toByteArray() &&
 {
+    // ### Replace with std::move(chunk).sliced(head(), size()) once sliced()&& is available
     if (headOffset != 0 || tailOffset != chunk.size()) {
         if (isShared())
-            return chunk.mid(headOffset, size());
-
-        if (headOffset != 0) {
-            char *ptr = chunk.data();
-            ::memmove(ptr, ptr + headOffset, size());
-            tailOffset -= headOffset;
-            headOffset = 0;
-        }
+            return chunk.sliced(head(), size());
 
         chunk.resize(tailOffset);
+        chunk.remove(0, headOffset);
     }
 
     return std::move(chunk);