Start a proof-of-concept FUSE interface to old snapshots.

Begin work on a FUSE interface to cumulus, allowing old snapshots to
displayed as a mounted filesystem.  Though only partly-implemented, already
it is possible to read the directory structure and stat information for
files.  File contents cannot yet be extracted.

To implement this efficiently, random access to cumulus metadata was
implemented through the use of a binary search.  Some optimization is still
needed, and some caching should probably still be added.