Fix pg_mcv_list_items() to produce text[]

The function pg_mcv_list_items() returns values stored in MCV items. The
items may contain columns with different data types, so the function was
generating text array-like representation, but in an ad-hoc way without
properly escaping various characters etc.

Fixed by simply building a text[] array, which also makes it easier to
use from queries etc.

Requires changes to pg_proc entry, so bump catversion.

Backpatch to 12, where multi-column MCV lists were introduced.

Author: Tomas Vondra
Reviewed-by: Dean Rasheed
Discussion: https://postgr.es/m/20190618205920.qtlzcu73whfpfqne@development

src/test/regress/expected/stats_ext.out

@@ -614,9 +614,9 @@ SELECT m.*
        pg_mcv_list_items(d.stxdmcv) m
  WHERE s.stxname = 'mcv_lists_stats'
    AND d.stxoid = s.oid;
- index |  values   |  nulls  | frequency | base_frequency 
--------+-----------+---------+-----------+----------------
-     0 | {1, 2, 3} | {f,f,f} |         1 |              1
+ index | values  |  nulls  | frequency | base_frequency 
+-------+---------+---------+-----------+----------------
+     0 | {1,2,3} | {f,f,f} |         1 |              1
 (1 row)
 
 -- mcv with arrays