pack-objects: move 'layer' into 'struct packing_data'

pack-objects: move 'layer' into 'struct packing_data'

This reduces the size of 'struct object_entry' from 88 bytes
to 80 and therefore makes packing objects more efficient.

For example on a Linux repo with 12M objects,
`git pack-objects --all` needs extra 96MB memory even if the
layer feature is not used.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>