Cube extension kNN support

Hello, hackers.

Here is the patch that introduces kNN search for cubes with euclidean, taxicab and chebyshev distances.

Following distance operators introduced:

<#> taxicab distance
<-> euclidean distance
<=> chebyshev distance

For example:
SELECT * FROM objects ORDER BY objects.coord <-> '(137,42,314)'::cube LIMIT 10;

Also there is operator "->" for selecting ordered rows directly from index.
This request selects rows ordered ascending by 3rd coordinate:

SELECT * FROM objects ORDER BY objects.coord->3 LIMIT 10;

For descendent ordering suggested syntax with minus before coordinate.
This request selects rows ordered descending by 4th coordinate:

SELECT * FROM objects ORDER BY objects.coord->-4 LIMIT 10;

Stas Kelvich.