core: Clean up object storage API

core: Clean up object storage API

Rather than passing xattr/file_info for all objects, change the API to
assume we're passing the defined object stream for each type.  Namely,
for OSTREE_OBJECT_TYPE_FILE, we're now giving the "archive file" data.

This significantly cleans up the code for committing to archive mode
repositories, at the cost of having to (at present) create an
intermediate temporary file when committing to raw repositories.