Index Only Scan support for cube

Hi, hackers!

Here's a small patch that implements fetch function necessary for
Index Only Scans that use cube data type.
I reuse function g_cube_decompress() instead of creating new function
g_cube_fetch().
Essentially, they both have to detoast data.

How do you think, is it better to create a shallow copy of
g_cube_decompress instead?
Any other suggestions on the functionality?

This

CREATE TABLE SOMECUBES AS SELECT CUBE(X,X+1) C FROM GENERATE_SERIES(1,100) X;
CREATE INDEX SOMECUBES_IDX ON SOMECUBES USING GIST(C);
SET ENABLE_SEQSCAN = FALSE;
EXPLAIN (COSTS OFF ) SELECT C FROM SOMECUBES WHERE C<@CUBE(30,40);

now produces

Index Only Scan using somecubes_idx on somecubes
Index Cond: (c <@ '(30),(40)'::cube)

instead of

Index Scan using somecubes_idx on somecubes
Index Cond: (c <@ '(30),(40)'::cube)

Best regards, Andrey Borodin, Octonica.