Added 64-bit extensions, comments, extended statistics 
and trace prints.


include/my_base.h:
  Bug#8321 - myisampack bug in compression algorithm
  Added a comment.
myisam/mi_packrec.c:
  Bug#8321 - myisampack bug in compression algorithm
  Fixed a function comment.
myisam/myisampack.c:
  Bug#8321 - myisampack bug in compression algorithm
  Enlarged the variables which hold Huffman codes to
  ulonglong and adjusted the functions accordingly.
  Added test code for long Huffman codes.
  Enlarged the distinct column values buffer (tree_buff)
  and added checks to stay in its range.
  Added statistics and trace prints.
  Added a lot of comments.
mysys/tree.c:
  Bug#8321 - myisampack bug in compression algorithm
  Added a check against overflow of the tree element count.
  The tree element count is only 31 bits, but sometimes
  used for big numbers. There is however no application yet,
  which relies on exact tree element counts.