Changes in amcanreturn() interface to support multicolumn indexes

- GIST. In first version it works only for onecolumn indexes. gistcanreturn() checks whether fetch() method is defined for column's data type.

There is a question of support multicolumn index only scans for GIST.

gistcanreturn() can return true if fetch is implemented for all indexed columns and false otherwise.

But that's not very good case for multicolumn indexes.

I think, it requires extend the interface to add separate columns checking.

But I can't understand what kind of changes is required 

and how would it affect on previous amcanreturn interface.