LP2041364: Attempt to speed up 852 insertion in marc_export

LP2041364: Attempt to speed up 852 insertion in marc_export

Instead of using MARC::Record->insert_grouped_field on the list of all
item tags (852), we now do insert_grouped_field only on the first item
tag and then use MARC::Record->insert_fields_after for the remainder
of the list. According to some testing this can provide a 78% speed
increase when adding item tags to records with many items.

Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Jane Sandberg <js7389@princeton.edu>