Optimize sort and deduplication in ginExtractEntries()

Optimize sort and deduplication in ginExtractEntries()

Remove NULLs from the array first, and use qsort to deduplicate only
the non-NULL items. This simplifies the comparison function. Also
replace qsort_arg() with a templated version so that the comparison
function can be inlined. These changes make ginExtractEntries() a
little faster especially for simple datatypes like integers.

Author: David Geier <geidav.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/6d16b6bd-a1ff-4469-aefb-a1c8274e561a@iki.fi