pack-bitmap.c: keep track of each layer's type bitmaps

pack-bitmap.c: keep track of each layer's type bitmaps

Prepare for reading the type-level bitmaps from previous bitmap layers
by maintaining an array for each type, where each element in that type's
array corresponds to one layer's bitmap for that type.

These fields will be used in a later commit to instantiate the 'struct
ewah_or_iterator' for each type.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>