New feature:
   1. Support of variable size keys - new algorithm of insertion to tree
      (GLI - gist layrered insertion). Previous algorithm was implemented
      as described in paper by Joseph M. Hellerstein et.al
      "Generalized Search Trees for Database Systems".  This (old)
      algorithm was not suitable for variable size keys and could be
      not effective ( walking up-down ) in case of multiple levels split
Bug fixed:
   1. fixed bug in gistPageAddItem - key values were written to disk
      uncompressed. This caused failure if decompression function
      does real job.
   2. NULLs handling - we keep NULLs in tree. Right way is to remove them,
      but we don't know how to inform vacuum about index statistics. This is
      just cosmetic warning message (like in case with R-Tree),
      but I'm not sure how to recognize real problem if we remove NULLs
      and suppress this warning as Tom suggested.
   3. various memory leaks

This work was done by Teodor Sigaev (teodor@stack.net) and
Oleg Bartunov (oleg@sai.msu.su).