Reduce size of PackedObjectLoader by dropping long to int

Reduce size of PackedObjectLoader by dropping long to int

Rather than keep track of both the position of the object, and the
position of its data, just keep track of the number of bytes used
by the object's header in the pack.  This shaves 4 bytes out of the
size of the PackedObjectLoader instances.

We also can defer the addition instruction to the materialize()
operation, avoiding it entirely if the caller never actually uses
the loader.  This may be relevant for PackWriter invocations,
where only 1 loader gets chosen for a given object, even though
the object may appear on disk in more than one pack file.

Error reporting is now simplified, as we can rely on the object
offset rather than its data offset.  This is the value displayed
by pack debugging tools like `git verify-pack -v`, so its better
to use that in our own errors.

Because nobody needs getDataOffset() now, we can drop that from
the public API.

Change-Id: Ic639c0d5a722315f4f5c8ffda6e26643d90e5f42
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>