Cleanup covering infrastructure

- Explicitly forbids opclass, collation and indoptions (like DESC/ASC etc) for
  including columns. Throw an error if user points that.
- Truncated storage arrays for such attributes to store only key atrributes,
  added assertion checks.
- Do not check opfamily and collation for including columns in
  CompareIndexInfo()

Discussion: https://www.postgresql.org/message-id/5ee72852-3c4e-ee35-e2ed-c1d053d45c08@sigaev.ru

src/backend/utils/adt/selfuncs.c

@@ -7437,6 +7437,8 @@ gincost_pattern(IndexOptInfo *index, int indexcol,
 	int32		searchMode = GIN_SEARCH_MODE_DEFAULT;
 	int32		i;
 
+	Assert(indexcol < index->nkeycolumns);
+
 	/*
 	 * Get the operator's strategy number and declared input data types within
 	 * the index opfamily.  (We don't need the latter, but we use