WIP: Fast GiST index build

Hackers,

WIP patch of fast GiST index build is attached. Code is dirty and comments
are lacking, but it works. Now it is ready for first benchmarks, which
should prove efficiency of selected technique. It's time to compare fast
GiST index build with repeat insert build on large enough datasets (datasets
which don't fit to cache). There are following aims of testing:
1) Measure acceleration of index build.
2) Measure change in index quality.
I'm going to do first testing using synthetic datasets. Everybody who have
interesting real-life datasets for testing are welcome.

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With best regards,
Alexander Korotkov.