[core] Add global option to flatten when appending in XmlStringBuilder

For certain use cases, this provides a performance improvement,
probably due better cache locality. However, it comes with the cost of
additional memory consumption.

This was initially suggested by Boris Grozev, who also reported a
significant performance problem of
XmlStringBuilder/LazyStringBuilder. However, the main cause of the
performance probelm was the missing caching of LazyStringBuilder. The
length of the lazy string is now cached by LazyStringBuidler since
70e4830 ("[core] Cache length in LazyStringBuilder"), which
accounts for large performance improvement. A significantly smaller
improvement is achieved by this commit and setting
XmlStringBuilder.FLAT_APPEND to 'true'.

Suggested-by: Boris Grozev <[email protected]>

smack-core/src/main/java/org/jivesoftware/smack/util/XmlStringBuilder.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2014-2021 Florian Schmaus
+ * Copyright 2014-2023 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,6 +33,8 @@
 public class XmlStringBuilder implements Appendable, CharSequence, Element {
     public static final String RIGHT_ANGLE_BRACKET = Character.toString('>');
 
+    public static final boolean FLAT_APPEND = false;
+
     private final LazyStringBuilder sb;
 
     private final XmlEnvironment effectiveXmlEnvironment;
@@ -596,7 +598,17 @@ public XmlStringBuilder condAttribute(boolean condition, String name, String val
     @Override
     public XmlStringBuilder append(CharSequence csq) {
         assert csq != null;
-        sb.append(csq);
+        if (FLAT_APPEND) {
+            if (csq instanceof XmlStringBuilder) {
+                sb.append(((XmlStringBuilder) csq).sb);
+            } else if (csq instanceof LazyStringBuilder) {
+                sb.append((LazyStringBuilder) csq);
+            } else {
+                sb.append(csq);
+            }
+        } else {
+            sb.append(csq);
+        }
         return this;
     }