chunked: optimize mergeTocEntries
chunked: optimize mergeTocEntries

avoid using slices.  I've seen a drop of ~20M in memory
usage with a fedora image.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

containers/storage,containers/storage,containers/storage