optimization when freeing hash tables. some tuning of buffer allocations

* dict.c hash.c: optimization when freeing hash tables.
* parser.c xmlIO.c include/libxml/tree.h: some tuning of buffer
  allocations
* parser.c parserInternals.c include/libxml/parser.h: keep a
  single allocated block for all the attributes callbacks,
  avoid useless malloc()/free()
* tree.c: do not realloc() when growing a buffer if the buffer
  ain't full, malloc/memcpy/free avoid copying memory.
Daniel

include/libxml/tree.h

@@ -50,7 +50,7 @@ typedef xmlEntity *xmlEntityPtr;
  *
  * default buffer size 4000.
  */
-#define BASE_BUFFER_SIZE 4000
+#define BASE_BUFFER_SIZE 4096
 
 /**
  * XML_XML_NAMESPACE: