* 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