New version of gist.c for PostgreSQL 7.1

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

All our tests and Gene Selcov's regression tests passed ok.
We have version also for 7.0.3
Sample application which utilize RD-Tree for index support of
int arrays is in contrib/intarray (will be submitted separately).

TODO:

1. Description of GLI algorithm
2. regression test for GiST (based on RD-Tree)

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