[PATCH] Add array_reverse() function

Hi,

Recently I wanted to call array_reverse() and discovered that we still
don't have it. I'm not the first one who encountered this limitation.
array_reverse() was requested at least since 2009 [1] and the
workaround on PostgreSQL Wiki is dated 2010 [2].

The proposed patch adds this function. Only the first dimension of the
array is reversed, for consistency with the existing functions such as
array_shuffle() [3].

Examples:

=# SELECT array_reverse(ARRAY[1,2,3,NULL]);
array_reverse
---------------
{NULL,3,2,1}

=# SELECT array_reverse(ARRAY[[1,2],[3,4],[5,6]]);
array_reverse
---------------------
{{5,6},{3,4},{1,2}}

Thoughts?

[1]: http://postgr.es/m/4AEE80FC.7010608%40postnewspapers.com.au
[2]: https://wiki.postgresql.org/wiki/Array_reverse
[3]: https://www.postgresql.org/docs/current/functions-array.html

--
Best regards,
Aleksander Alekseev