Snapshot format change: extend the slice syntax with a length-only form.

Change slice format so that in addition to <start>+<length>, it is possible
to specify just <length>.  This isn't needed (0+<length>) could be used
instead, but looks more pleasing if lengths are specified more frequently
on objects.  Also update the various tools to correctly parse the new
syntax.

This is part of the new v0.6 format.