Since start is always the next element there is no need to explicitly
save it.  This commit also makes the remaining sub member relative to
the node, i.e. instead of saving the absolute offset into the array,
we instead just save the size of the whole subtree.

They probably should not get saved anyways!

We have to save the information on whether a file is the "original"
file or if its just a "link".  To do that we add the offset to the
original to every hardlinked file so that we can detect the actual
original by checking whether the node points to itself.

We need to ensure to free the build restore tree (if it exists) even
if we decide not to do a restore.

We now use the cache directory from the director configuration.

This class is easy to use if you only want to calculate simple checksums.

Since elements of type TN_NEWDIR *always* have a deltaseq of -1 we can
never not save them (since not all of them are 0).  To remedy this, we
instead save the delta_seq + (type == TN_NEWDIR), so that the vector
is all 0, even if we have NEWDIRs.