Tighten up local packed object representation during packing
Rather than making a loader, and then using that to fill the object
representation, parse the header and set up our data directly.
This saves some time, as we don't waste cycles on information we
won't use right now.
The weight computed for a representation is now its actual stored
size in the pack file, rather than its inflated size. This accounts
for changes made when the compression level is modified on the
repository. It is however more costly to determine the weight of
the object, since we have to find its length in the pack. To try and
recover that cost we now cache the length as part of our ObjectToPack
record, so it doesn't have to be found during the output phase.
A LocalObjectToPack now costs us (assuming 32 bit pointers):