next:
- clean up node bitfield implementation
- "insert" API for HyperRegister
- crude single-thread drive+network clone
- sparse register clone
- "needed data chunks" and progress function?
- crude single-thread drive+network broadcast
- 'geniza clone' command
- 'geniza checkout' command
- 'geniza init', 'geniza commit' commands

specific tests:
- drive children array: deep directory nesting (like 'tree' example)
- sleep info a variety of bad sleep files, should error
- sleep create a known file, should be same as empty (also a "Rot13" algo file)
- create a register, binary compare against known-good
- test vectors of leaf, parent, and root hashes
- test vectors of signatures
- cloning with metadata pointing to relative or absolute paths
  ("dir/../../../../etc/passwd")

performance:
- cache SLEEP file length when opened (don't fstat all the time)
- keep an LRU cache of SLEEP reads

Backburner:
- API to link and run from, eg, python
- uTP transport
- in-memory storage backend
- WASM
- multi-connection network sync (per-connection state, etc)
- duplicate file/chunk optimizations
- secret_key-in-home helpers (read/write)
- SLEEP bitfields stuff
- .latest and .ogd files
- benchmarks! syscall counts (strace), timing, etc
- test fault injection... with strace?
- consider kafka (append performance) and sqlite (robustness) file I/O patterns
- manpage
- slog for logging
- disable sodiumoxide serde feature (faster/smaller build)
- sodiumoxide static link
    https://safenetforum.org/t/building-a-non-dynamically-linked-executable-of-safe-vault/9811/6
- SLEEP API: should just be empty string for algo-name, not Option<>
- I have the SLEEP/register naming wrong... sleep refers to the directory?
- portable positional file I/O
    https://github.com/vasi/positioned-io
- mmap
    https://docs.rs/scroll/0.7.0/scroll/
- switch to byteorder for endian reads/writes
- --json args to most CLI commands
- ruthlessly simplify APIs
- SleepFile from existing File (api)
- refactor key generation into generic/re-usable function
- turn magic numbers in tree nodes into declared (public?) constants
- root_nodes as an iterator? (vs returning a Vec)
- pretty terminal output: https://crates.io/crates/colored
- error handling best practices:
    https://rust-lang-nursery.github.io/rust-cookbook/basics.html#obtain-backtrace-of-complex-error-scenarios
- debug commands with hex i/o:
    https://github.com/ia0/data-encoding
- sendfile() or splice() for disk-to-disk copying?