odb/source-loose: wire up `read_object_stream()` callback

odb/source-loose: wire up `read_object_stream()` callback

Move `odb_source_loose_read_object_stream()` and its associated helpers
from "object-file.c" into "odb/source-loose.c" and wire it up as the
`read_object_stream()` callback of the loose source.

As part of the move we are also forced to expose a couple of functions
from "object-file.h" that parse object headers in a somewhat-generic
way, as those functions are now used by both subsystems.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>